The term mosher comes from one that partakes in a mosh pit (the front of a concert a bit of pushing and shoving/dancing)
When this word started out, it was those people that liked there Rave music and it people who had long hair and liked Nirvana was branded as a mosha. Back then (1992-1998)Anyone who was branded a mosha was a minority and Ravers were the majority. This sort of battle has been going for years. Watch Quadrophinia - Mods & Rockers.
However, the younger generation (that were born after 1983) think it is a great word for someone who likes "Nu Metal" And go around calling people fake moshas (or as the spelling was originally "Moshers".