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Jack/ˈdʒæk/ is a male given name, although in some cases it can be used as a female given name (a shortened versioned of "Jacqueline" or "Jackie", for example), and sometimes as a surname. In English it is traditionally used as the diminutive form of the name John, though it is also often given as a proper name in its own right.
The name Jack is unique in the English language for the frequency of its use as a verb and a noun for many common objects and actions; and also its use in many compound words and phrases: jack-of-all-trades, jackknife, jackpot, jack tar, hijack, union jack, jack straw (scarecrow), apple jack, lumberjack, jackhammer, jackdaw, jack o'lantern, jack-off, jack-in-the-box, jack of clubs (playing cards), etc. The Encyclopædia Britannica article on the history of the word "jack" linked it directly to the common name: "Jack, a word with a great variety of meanings and applications, all traceable to the common use of the word as a by-name of a man." 
Jack is thought to harken back to medieval times as a common derivative of the name John. The nickname has also been seen as Jackie among men (examples of note: Jackie Gleason; Jackie Coogan, Jackie Cooper, all of whom were formally named John). This "Jackie" diminutive of the nickname Jack is thought by some to have hailed from the earlier "Jackin" (born of the appellation Jankin). Alternatively it may be derived from the name Jacques, which is the French form of the name Jacob. There is also a theory that it is Celtic in origin, meaning "Healthy, Strong, Full of Vital Energy" (compare the Welsh word iach, "health"), from a putative Ancient British*Jakkios. Whatever its origin, the name and also the word "jack" were long used as a term to refer to any man, especially of the common classes.
In recent years (most notably since 2003), the nickname Jack (for the more formal John and, to a lesser extent, for Jacob) has become increasingly used as the actual formal name (ie., baptismal name, vs a nickname) for boys in England, Wales, Northern Ireland, and the United States. Jack was included on lists of the most frequently used male baby names in the UK for the years 2003–2007. The high status of Jack as a formal (vs it traditional John-derivative) male given name is a recent phenomenon, although the formal name from which it derives - John - has held the title, along with William of most frequent male name throughout the 1880s and first half of the 1900s. For all of the 1800s and the first half of the 1900s, William or John held the title of most frequent male given name. A closer look at London shows that in 1994 "Jack" was the most popular name in the city, but does not appear among the top-ten most popular in the preceding 970 years.[not in citation given] A survey in December 2008 showed that Jack is the most popular name in the UK for the 14th year running, since it took over from Thomas in 1994.
According to the United States Census of 1990, "Jack" is an uncommon American name, the given name for 0.315% of the male population and 0.001% of the female population. However, the frequency of use of "Jack" as a baby name has steadily increased from the 160–170 rank prior to 1991 to a rank of 35 in 2006.
Jock is the Scottish form of the name John. It's similarity to the common John derivative/nickname Jack is apparent.
The appearance of "Jack" as a surname is much less frequent than its use as a male given name. In 1990 United States, this surname is shared by about 0.007% of the population, though the geographical distribution of the surname has been broad since at least 1840, at which time there was a modest concentration of Jacks in Pennsylvania. In the late 19th century, the geographic distribution of Jacks in England was also broad, with enrichment in North East England, Yorkshire and the Humber and southern North West England. In 1891, the highest concentration of Jacks in the United Kingdom appeared in Scotland. The surname is considered almost exclusively Scottish.
Jack (a.k.a. Subject Zero), a squad member from the Mass Effect series. It is worth noting that she is female, and "Jack" is merely a nickname. Her real name is revealed to be Jennifer in Mass Effect 3.