Synonymous with the word 'fuck.' Notably used in cases where the writer does not know (American) English well and in attempt to sound out the word while applying his known letter/pronunciation set results in a slightly mangled but understandable version of the word.
Example at engrish.com, where some silly Japanese bastard decided to spraypaint profanities on the wall yet failed in his (American) English spelling:
Fack You Man

(alternately pronounced as 'fack', similar to the word 'fact', to those who find such objects of Japanese word scrambling humorous and to explicitly specify that you are not saying the word 'fuck', but rather the word 'fack')
by Stimar May 7, 2003
A prolonged way of saying "fuck" when you are busted in an awkward/uncomfortable situation.
Timmy: "Chaz shit his pants yesterday."

Shelly: "He's sitting right behind you..."

Timmy: "Fack..."
by Mere Scraps August 3, 2009
Used in Bermuda. Replacement for the word "fuck".
by Heather B. May 16, 2008
The way old Asian ladies say fuck.
Ling Ling what da fack are yu doing!?!
by queengooz March 3, 2015
Romanian term for do, however, is the American term for Romanian's term for fuck.
You can fack it!
by versatil December 23, 2003
When abusing someone in the heat of the moment, you can't decide whether to call them a fag or a fuck. So you call them a fack.
Chopz: i hope hot chicks are provided as well as relivant materials. ie gloves
Ken: Gloves
Ken: WTF
Ken: YOU WEAK FACK
by Ken June 13, 2004