That one friend you have who is fundamentally a bitch. This is the lowest form of friend and should be rediculed mercilessly when acting like a bitch towards another friend.
This is the origin of the word as far as I know. Two opposing ramps originally used for BMXers, but now used by snow boarders, roller bladers, skateboards and others. See also quarter pipe.
I heard they're building a new half-pipe down at the park this summer. Let's try it out but I'm not down with wearing all of the armor they are going to require.