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!