Double-jointed: Popular term to describe a joint that is unusually flexible. Medically, the joint is said to be hyperflexible, hyperextensible, or hypermobile. People whose fingers are hypermobile have higher rates of arthritis in the hands.
[adj] - having unusually flexible joints especially of the limbs or fingers
adjective having unusually flexible joints especially of the limbs or fingers
(of particular people or animals) having unusually flexible joints that can bend in unusual ways or to abnormally great extent.