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')