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!