Joke is a legal term denoting a common defense for violent criminal acts.
Joke arises from old
English Common Law, when it was common to
kill a man, shrug, and tell the Sheriff of Nottingham, "We were just joking and the clumsy prick fell onto that
knife I left in his back."
The Sheriff was required by Common Law to
flick the criminal's ear and tell him, "It's funny until someone loses an eye." In extreme cases, the Sheriff would require the criminal to go stand in the corner for fifteen minutes, after which he would chide the criminal, "It's not so funny now, is it dumbass?"