Risible
Laughable, but in a negative sense. You wouldn't call a comedian's jokes risible if you liked them. If he was a painfully bad comedian, you could say that his attempt at comedy was risible. This is not a common word in modern English. It usually only shows up in pompously written book or movie reviews or political essays - the sort of thing written by people who can't see that their stuffy prose isn't admirable but is instead risible. The word appeared in English in the 1500s. Back then, it meant able to laugh, capable of laughing. By the 1700s, it had come to mean evoking laughter, laughable, but it didn't have a negative connotation yet. That's more modern. The root is the Latin word ridere, to laugh. Our word "deride" comes from the Latin combination de (down) combined with ridere. Someone who uses "risible" in ordinary speech is likely to encounter derision.
(Will be published in the August 2010 issue of Denver's Community News.)
I'm collecting all of these at:
http://www.dvorkin.com/davidsdefs.html