When someone jumps in on a joke or a piss-take and trys to add to it or take it over. Normally a sad person who has no life or the usual cunt who just needs attention that othwise he/she would go un-noticed.
1st person: Hey did you hear the one about...etc,etc
Sad lonely cunt: Haha yeh ive heard this (then trys to take over joke or adds to it.
1st person: Oh great, a Joke Jacker, what a cunt