This article is from the Stand-up Comedy FAQ, by Steven Rosenthal and Steve Silberberg (firstname.lastname@example.org) with numerous contributions by others.
A. Stolen material. There's nothing lower or more reprehensible than
someone who steals your material - - especially if they do it better than
you. A hack also refers to material done to death, e.g. Cops in Donut
shops, 7-11 jokes, Clinton-Lewinski jokes, Viagra jokes, differences
between cats and dogs, airline food, etc. etc.
There is an entire FAQ, called the "Hack FAQ"
(http://rampages.onramp.net/~stevebo/hack.html) which gets posted
periodically to alt.comedy.standup and was originally written by Steven