Unlike certain great, eternal rock mysteries ("What is glam?" or "Who invented the cowbell?"), the question "What is emo?" is destined for a Trivial Pursuit afterlife at best. That's because the term "emo" denotes a murky feeling-tone more than a musical genre and the music it does represent is, for the most part, unoriginal and annoying. Emo is a pseudo-genre so ill conceived, it makes other dodgy umbrella terms like "electro-clash" look positively descriptive by comparison.
The term comes from "emo-core," short for "emotional hardcore" which is, verily, the most retarded name ever cobbled for a musical genre. Various self-appointed emo experts tell me that the whole emotional-hardcore thing is basically Fugazi's fault. (All that earnest hollering and lyric writing and van touring and no-drug-taking and hairline receding, dontcha know. They weren't the first, but the most famous.)
If only emo began and ended with DC hardcore. Unfortunately, part of the problem with defining emo is that as hardcore melted into post-hardcore, "emo" got passed around like mono. The term changed hands so many times over the years, it eventually mutated to mean just about anything sung by a whiny white guy with sneakers and no sense of humor.
Source: LA Weekly