Someone with a joint that abnormally bends in two directions, rather than one. For example, I'm double-jointed in my fingers, and can bend them backwards until my fingertips touch the back of my hand. I've known people who can bend their elbows backwards, and supposedly some people can do the same with their knees. The term is something of a misnomer, since we don't actually have an extra joint.

Back in middle school, being double-jointed was useful for freaking out girls and pretending that I'd gotten my hand slammed in a door. Nowadays, other than playing tricks on stoned friends, the best thing I can do with my talent is hold four soda cans in a single hand, and sometimes when people offer me a handshake, shake their right hand with my left.