In slang, a Mickey Finn (or simply Mickey) is a drink laced with a drug (especially chloral hydrate) given to someone without their knowledge in order to incapacitate them. Serving someone a Mickey is most commonly referred to as slipping one a mickey, sometimes spelled "slipping one a mickie".
The Mickey Finn is most likely named for the manager and bartender of a Chicago establishment, the Lone Star Saloon and Palm Garden Restaurant, which operated from 1896 to 1903 in the city's South Loop neighborhood on South State Street. In December 1903, several Chicago newspapers document that a Michael "Mickey" Finn managed the Lone Star Saloon and was accused of using knockout drops to incapacitate and rob some of his customers. Moreover, the first known written example (according to the Oxford English Dictionary) of the use of the term Mickey Finn is in 1915, twelve years after his trial, lending credence to this theory of the origination of the phrase.
The first popular account of Mickey Finn was given by Herbert Asbury in his 1940 book Gem of the Prairie: An Informal History of the Chicago Underworld. His cited sources are Chicago newspapers and the 1903 court testimony of Lone Star prostitute "Gold Tooth" Mary Thornton. Before his days as a saloon proprietor, Mickey Finn was known as a pickpocket and thief who often preyed on drunken bar patrons. The act of serving a Mickey Finn Special was a coordinated robbery orchestrated by Finn. First, Finn or one of his employees, which included "house girls", would slip a drug (chloral hydrate) in the unsuspecting patron's drink. The incapacitated patron would be escorted or carried into a back room by one of Finn's associates who would then rob the victim and dump him in an alley. Upon awaking the next morning in a nearby alley, the victim would remember little or nothing of what had happened. Finn's saloon was ordered closed on December 16, 1903.
In 1918, Mickey Finn was apparently arrested again, this time for running an illegal bar in South Chicago.
Chicago restaurant poisonings
On June 22, 1918, four people were arrested and over one hundred waiters taken into custody over the apparent widespread practice of poisoning by waiters in Chicago. Guests who tipped poorly were given "Mickey Finn powder" in their food or drinks. Chemical analysis showed that it contained antimony potassium tartrate, well known emetic drug that besides vomiting can cause headaches, dizziness, depression, and can be lethal in large quantities. W. Stuart Wood and his wife were arrested for manufacturing the powder, and two bartenders were arrested for selling the powder at the bar at the waiters' union headquarters. Wood sold packets of the powder for 20 cents  and referred to it as "Mickey Finn Powder" in a letter to union bartender John Millian. A follow-up article mentions the pursuit of a man named Jean Crones who was believed to be responsible for poisoning over 100 people at a Chicago University Club banquet at which three people died.
Tracing usage of the phrase "Mickey Finn"
The Oxford English Dictionary gives a chronology of the term, starting in 1915. The 1915 citation is from a photograph of a saloon in the December 26 edition of the Los Angeles Examiner. In the photograph is a sign that reads "Try a Michael Finn cocktail". The first listed reference as a knock-out drop in the OED, "Wish I had a drink and a Mike Finn for him", is from a March 11, 1924 article in the New York Evening Journal. A description of a Mickey Finn is given in the January 18, 1927 issue of the Bismarck Tribune, "a Mickey Finn is an up-to-date variant on the knock-out drops of pre-war days". In the September 3, 1927 issue of the Chicago Daily Tribune, the phrase appears in an article on the use of ethylene for artificial ripening of fruit, "Applied to a human, ethylene is an anaesthetic as the old-time Mickey Finn in a lumber-jack saloon". The phrase also appears in the January 13, 1928, issue of Variety, "Mickeyfinning isn't describable, but it's easily worked, leaving its victims miserable. The work is accomplished mainly by bartenders... Mickeyfinning has been behind some of the nite club liquor trouble, with the victims so sore they don't care what their revenge might bring".
^The saloon's exact location is usually said to be the on west side of South State Street, just north of Congress Parkway. The entire west side of South State Street, between Congress and Van Buren, is now occupied by Chicago Public Library's Central Library (also known as the Harold Washington Library at 400 South State Street). The December 16–17, 1903, Chicago Daily Tribune articles give the address as 527 State Street (Corner of State and Harmon Court), however, which is now the 1100 block of South State Street. The 500 block of South State Street now is between Congress Street and Harrison Street, which may be the reason for the confusion of the saloon's location. For the 1903 Chicago street names and numbering refer to "New map of Chicago showing street car lines in colors and street numbers in even hundreds", Chicago : Rufus Blanchard, 1897.
^The area on State Street centered between Van Buren Street (to the North) to Harrison Street (to the South) was known as "Whiskey Row" from the late 1880s to the early 1910s. Just south of Harrison Street was a block known as "Hell's Half Mile". The area of State Street, south of Harrison was also known as "Satan's Mile".
^"GRAFT EVIDENCE USED TO INDICT; Stories of Knockout Drops", Chicago Daily Tribune, December 16, 1903: 5. Quoting from the article, "Michael Finn, owner of saloon at corner of State Street and Harmon Court [now East 11 Street] ... Two former habitués—Mrs. Mary Thornton and Isabelle Fyffe—told that he gave 'knock-out drops' to customers suspected of having money and afterwards robbed them." Mary Thornton is quoted, "I worked for Finn a year and a half and in that time I saw a dozen men given 'dope' by Finn and his bartender. The work was done in two little rooms adjoining the palm garden in back of the saloon".
^"Graft Fighters Win Two Battles; License for Finn's Resort Is Revoked and One Saloon keeper Indicted", Chicago Daily Tribune, December 17, 1903: 3. Quoting from the article, "Lone Star Saloon, 527 State Street [now 1100 block of South State Street], managed by Micky Finn, closed by order of Mayor Harrison."
^Chicago Daily News, December 16, 1903. "The complete defense advanced by 'Mickey' Finn, proprietor of the Lone Star saloon ... described ... as the scene of blood-curdling crimes through the agency of drugged liquor."
^Inter-Ocean [Chicago], December 17, 1903. The Inter-Ocean was another Chicago newspaper in 1903. "Lone Star Saloon loses its license. 'Mickey' Finn's alleged 'knock-out drops' ... put him out of business."
^"EXTRY! EXTRY! THEY'VE TAKEN MICKEY FINN", Chicago Daily Tribune, July 8, 1918: 1. "Mickey Finn was arrested last night and lodged in the South Chicago police station. Mickey also known as Mike runs a hut at 115th Street and the Calumet River. He and his housekeeper Millie Schober and twenty customers were swooped down on by the police and all taken to the station. A wagon load of beer and booze was confiscated. Mickey and the woman were charged with running a disorderly house and selling liquor without formal authorization..."
^"Drugs to the Non-Tippers Arrested Chicago Waiters Confess Poisoning Hotel Guests. Detective Seize Large Quantity", The Kansas City Times, June 23, 1918: 3 "Evidence against the waiters was obtained by a detective agency employed by the Hotel Sherman after several guests had become ill suspiciously...Large quantities were found in a drawer behind the bar at the waiters' union headquarters.
^"Charge Waiters Gave Poison to Tipless Diners Alleged Drug Maker, His Wife and Two Bartenders", Duluth News Tribune, June 24, 1918: 1
^ ab"WAITERS TAKEN FOR DRUGGING NONTIPPERS — Hoyne gets evidence of plot against Hotel Guests", Chicago Daily Tribune, June 23, 1918: 1
^"WAITER POISON VICTIMS NAMED TO POLICEMEN", Chicago Daily Tribune, June 24, 1918: 3
^"WAITERS DISOWN "MICKEY FINNS"; DENOUNCE HOYNE", Chicago Daily Tribune, June 25, 1918: 13
^"WOMEN TO BARE NEFARIOUS WORK OF "MICKEY FINN"", Chicago Daily Tribune, July 8, 1918: 7
^""MICKEY FINN" POWDER GIVEN TO PROSECUTOR — Michels, Hoyne Assistant gets dose while investigating case", Chicago Daily Tribune, August 7, 1918: 13
^"TEN INDICTED IN DIABOLISMS OF 'MICKEY FINN'", Chicago Daily Tribune, July 9, 1918: 8.
^"EVIDENCE GIVEN U. S. OF SALES OF "MICKEY FINNS"", Chicago Daily Tribune, July 11, 1918: 7. "Friend Johnny: Am enclosing two dozen packets of the Mickey Finn Powder...also find enclosed a couple hundred circulars...These circulars are not for use in Chicago...Whenever you have a man that is leaving Chicago talk Mickey Finn to him and give him a few of these circulars...
^"On Trail of Poisoner Authorities Believe Jean Crones, Wanted in Chicago, is in Army", The Idaho Daily Statesman, June 26, 1918: 5.
^"Looking for Jean Crones. Chicago Police Believe Chef Had Hand in "Micky Finn" Poison Case", The Kansas City Times, June 26, 1918: 5.
^"Synthetic sun to ripen fruit while its iced", Chicago Daily Tribune, September 3, 1927: 17
Herbert Asbury, Gem of the Prairie: An Informal History of the Chicago Underworld (New York: Alfred A. Knopf, 1940).
James A. lnciardi, "THE CHANGING LIFE OF MICKEY FINN: Some Notes On Chloral Hydrate Down Through the Ages", Journal of Popular Culture, Winter 1977 - Vol. 11 Issue 3 Page 591.