Joke-off means in slang: Joke-offs are when one person tells another joke. Usually, it is a bad joke. Another person counters the first with a joke. Next, another person responds with a joke. Ad infinium… Another person might make a comment during these proceeds. It becomes an open joke. Normal people, who are normal and sane, should flee from the area as soon as they can. You might also find that jokes lose their taste or decency very quickly after they are told. (in Slang Dictionary, added by Kamryn Shannon)
