I first heard someone use "diss" sometime in the mid-nineties, which probably means that it entered the vernacular a few years before that. I have the vague idea that it come out of Black American parlance as a shortening of "disrespect", but I've never seen this looked into seriously. It's a good, useful, word, and what's surprising is the hole in the language that we didn't know was there until it was filled.
Conventional English doesn't really have a synonym for diss; I guess the closest would be "deprecate", but that has a formal usage in the worlds of standards and technologies that has come to be dominant, in my mind at least.
