Ah, 1987



So, Merriam-Webster's dates "mosh" back to 1987, and says it is a variant of "mash." I find this unsatisfying, but look at the definitions for yourself-- they don't vary much. The only question is if moshing is a synonym for slamdancing, or if slamdancing is more violent than moshing. I think in 1987 I would have cared.