From the theater: to laugh uncontrollably and not be able to stop. (Source: Cate Blanchette on the 12/04/06 Tonight Show). Wikipedia defines it as follows: "Corpsing is a theatrical slang term used to describe when an actor breaks character during a scene - usually by laughing or forgetting his or her lines, or by causing another cast member to do so. Though the origin of the term is unknown, it refers to almost literally murdering the scene.
A BBC TV program on 18 November 2006 stated an opinion that the term "corpsing" originated when there was a corpse on stage, acted by a living actor: there was sometimes a tendency to try to make that actor laugh."
"Hugo Weaving was in a production of King Lear and he could just sat there corpsing."