(pronounced an-tee craCK) a typical response to a person or person's actions which goes against the general trend of fun in the room. Originates in the Irish town of Derry where craic (meaning good times and general tomfoolery) is sacrosanct. The anti-craic is the metaphorical devil in said situation.
Example 1 (In Derryspeak):
"Deeks was all 'hi maccers de ye wannay head down the 720 fer a wee c/o before sugar the nite' and Maccers was all 'naw am already tappin me ma fer a sub' and Deeks was all 'you are the anti-craic'.
Example 2 (In plain English):
Young Daniel kindly quizzed his friend Maxwell on the possibility of enjoying a sociable drink before their night at the discotheque, however Maxwell, responsibly apprehensive of the idea, explained the fact that his drinking habits had, of late, left him in debt to his mother and any further spending would invariably be disadvantageous to himself. Daniel, disappointed at young Maxwell's renunciation of fun, expressed said disappointment with the colloquial expression of dissapointment - namely "anti-craic"