Someone who engages in outdoor pursuits/sports that revolve around mountains. Said pursuits/sports usually include but are not limited to trail running, mountain biking, snowboarding, mountain boarding, rock climbing and kayaking. Mountain surfers sometimes can be classified by the the type of vehicle they drive (aka Subaru with snowboard/ski, bike or kayak rack on top or a busted up Honda tattooed with mountainous sponsor's stickers) or simply by the way they talk and or dress.
"Dude, Chris Sharma just cranked out a 5.15b called "Jumbo Love"! Damn thing was nothing but crimpers with two massive slopers at the crux" -Mountain Surfer "Could you seriously put aside the mountain surfer lingo for a minute and rehash that?!" -average Joe
"So I slept in my Suby last night and was the first one to pull into Breck this morning. I seriously could NOT wait to shred mad powder! I'm gonna jib till I'm gassed!" -Mountain Surfer