MC - M/C - Motorcycle Club, referring to the tightly knit brotherhoods of biker gangs.
Mad Max - A circular burnout made by spinning the rear tire and then rotating the bike 360 degrees with the locked front wheel as the axis. Bonus for crossing the circle with a straight rubber mark when finished.
Mama - A woman who is available to all Biker Gang members
MANF - Multi Adjustable Nut F**ker, aka adjustable spanner! (UK). A wrench that messes up any bolt or nut it is applied to.
Market Street Commandos - An early Motorcycle Gang
Marquis deSaddle - A highly uncomfortable motorcycle seat
Masey Fergason/JCB/Tracker - Harley-Davidson
Mattress Cover or Ground Cover - Young Woman
Mechanic - Wrench
Megaphone - Goes onto the end of the exhaust to help tune it
Mexican Socket Set - Crescent wrench
Mill - Engine
Milwaukee Vibrator - A Harley-Davidson
Minger - Wheelie
MMI - Motorcycle Mechanics Institute.
Money Burner - Anyone who rides a Harley-Davidson
Monkey Butt - What you get after riding your dirt bike all day
Mono - Wheelie
Motorcycle Jockeys - Anyone who rides any motorcycle.
MSF - Motorcycle Safety Foundation (Training). The highly recommended way to learn how to properly and safely ride a motorcycle. Offered in many countries around the world for a very reasonable price.
Mud Puppies - ATV and ATC folks
Mushrooms or Crash Bungs - terms for the plastic 'bungs' you attach to the frame to protect the fairing etc in case of a 'spill' or crash.