Slang is a common socio-linguistic phenomenon, but how slang emerges and propagates is poorly understood. We explorethis problem by analyzing longitudinal data from 1,000 Reddit communities over the past decade. We consider socialand linguistic factors pertaining to the emergence and propagation of recently emerged online slang. We show that whilelinguistic factors can be relevant, social factors play a more important role in predicting the emergence and propagation ofonline slang. We find community size to be the dominant factor in the emergence of novel slang terms and user mobilityto be the most critical factor in the widespread propagation of slang.