*British* (participle), verb: to kef
A rather hard slap to the back of the neck.
NOTE: Does not mean, nor does it imply masturbation.
Dude 1: HOLY FUCK, JUSTIN BIEBER!
Dude 2: You deserve a mighty good keffing.
A new word on the streets meaning masturbation over ones own faeces to intoxication...
Boca was keffing over his mother.