Feck (or, in some senses, fek) is a monosyllable with several vernacular meanings and variations in Irish English, Scots and Middle English.
Vernacular usage of feck in the expletive sense is syntactically interchangeable with fuck, though it has no sexual connotations. This includes such phraseological variations as fecker (noun), fecking (verb or adjective), and feckin' 'ell. It can even be used to describe a person: "he's an old feck". It is not uncommon for school teachers and some members of the religious order to use the word 'feck' as an expletive in Ireland thus demonstrating the word's peculiarity in meaning to Ireland where it does not equate to the word 'fuck' as many people outside Ireland tend to think.
From the first sense we derive feckless, meaning witless, weak or ineffective; worthless; irresponsible; indifferent; lazy. Feckless remains a part of the Modern English and Scottish English lexicons; it appears in a number of Scottish adages:
In his 1881 short story Thrawn Janet, Robert Louis Stevenson invokes the second sense of feck as cited above:
Robert Burns uses the third sense of feck in the final stanza of his 1792 poem "Kellyburn Braes":
In a 1998 interview on Nickelodeon, Irish girl group B*Witched landed in hot water when a viewer complained one of the teenagers 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. 
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.
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 back 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!"