Slang expletive employed as an attenuated alternative (minced oath) to fuck or express disbelief, pain, anger, or contempt in a given situation, however it does not mean to have sex with in the same way that fuck does, and those aware of this use consider it a lesser expletive than fuck.
Verb meaning 'to steal' (e.g. 'They had fecked cash out of the rector's room.')
Verb meaning in Irish slang 'to throw' (e.g. 'He's got no manners at all. I asked him nicely for the remote control, and he fecked it across the table at me.')
Scots and Late Middle English
Feck (or fek) is a form of effeck, which is in turn the Scots form of effect. However, this Scots noun has additional significance:
"He had a feck o' books wi' him—mair than had ever been seen before in a' that presbytery..."
Robert Burns uses the third sense of feck in the final stanza of his 1792 poem "Kellyburn Braes":
I hae been a Devil the feck o' my life,
Hey, and the rue grows bonie wi' thyme;
"But ne'er was in hell till I met wi' a wife,"
And the thyme it is wither'd, and rue is in prime
Debate about the word's level of offensiveness
Magners Irish Cider have received complaints relating to an advert in which a man tells bees to "feck off": members of the public were concerned that young children could be badly influenced by seeing this advert. Magners claimed that the "feck off" mention in the advert was a "mild rebuff" to the bees, rather than an expletive. The Advertising Standards Authority (ASA) has ruled that the poster is suitable for display.
In a 1998 interview on Nickelodeon, Irish girl group B*Witched landed in hot water when a viewer made a complaint alleging that one of the teenagers had used the phrase "fuck off". Although Nickelodeon maintained that the singer had in fact said "feck off", which they described "a phrase made popular by the Channel 4 sitcom Father Ted," the item was found to be in breach of the ITC Programme Code and the complaint was thus upheld.
The word "feck" also frequently appears extensively in the Nintendo 64 video game Conker's Bad Fur Day, in which the word replaces "fuck" in all instances. It should be noted, however, that this substitution is in no way trying to limit cursing; the game itself is riddled with blatant swearing and innuendo. It is not known whether this substitution is due to the nationality of the producers or simply to give the game a lighter mood.
Heard repeatedly in YouTube videos featuring Loca, "the pug who can't run", and sung in lyrics with an Irish flair.
The word "feck" is also used in the movie Almost Famous by Cameron Crowe, as an alternative to the word "fuck". A teen girl (played by Zooey Deschanel) who is angry at her overbearing, strict mother (Frances McDormand) shouts, "Feck you!" Her mother is taken aback by this, stating aside to her 11-year-old son (Michael Angarano), "I can't believe she said the F-word," to which he replies, "I think she said 'feck'."
In the 1981 film Caveman, "feck" is used as a general term of disparagement. After Atouk's band of misfits defeats Tonda, the crowd joins in proclaiming "Tonda feck! Atouk! Atouk! ATOUK!" 
In 2004 French Connection UK, sellers of the popular "FCUK" T-shirt, won a legal injunction in Dublin that barred a local business from printing and selling a T-shirt marked "FCEK The Irish Connection".
The word "feck" appears in Frank McCourt's memoir Angela's Ashes and also the film which was based upon the book. English subtitles on the original DVD miscaption "feck" as "fuck".