A skateboarding/snowboarding trick that involves crazy amounts of rotation.
A McTwist consists of a 540 with a somersault. If it sounds tough to imagine somebody else doing, try doing it yourself. Invented in 1985 by Mike McGill, it started on vert, and is still only done there, though it's name has changed slightly. If you were to pull one off today, a grom or other bystander would say you pulled a 5, or a 540 if they wanted to say a lot.
If you try to do one off of a kicker on your snowboard, I believe it's called a rodeo flip or a misty flip instead, depending on the direction of rotation (correct me if I'm wrong). If you do one off a launch on your skateboard, you will break yourself.