This is "f u c k" spelled backwards. It is used to convey the emotion and intensity of the word "f u c k" without actually using it.
Mostly used when one does not want to physically SOUND vulgar, or offensive (
even though they intend to).
Used on websites such as
YouTube, which block offensive
words
Kcuf man, these guys can
dance!
Kcufing
hell, get a permit before you
post a video.
John: Ha ha, you didn't wipe your ass
Smith: Kcuf off!