Someone who steals, or poaches, other people's jokes and takes credit for the joke. Poaching usually occurs when a joke poacher heres another person tell a joke and repeats said joke in a louder more obnoxious way for all to hear.
John mumbled a joke under his breath. Will heard the joke and repeated it loudly in a crowded room. Everyone laughed at the hilarious joke and Will took credit for coming up with it. Will is a joke poacher.