"Unique and unable to classify"
Frank Zappa, the VW Beetle, cheese in a can: Sui generis refers to something that's so new, so bizarre, or so rare that it defies categorization. Granted, labeling something sui generis is really just classifying the unclassifiable. But let's not over-think it. Use it at a dinner party to describe Andy Kaufman, and you impress your friends. Use it too often, and you just sound pretentious.