If you are double-jointed, some of your joints can bend farther than most people’s — fingers that fold back toward the wrist, elbows or knees that look as if they bend a little the “wrong” way. It looks as if that body part had two joints working in different directions, which is how the word was built; there are not actually two joints. Some people do party tricks with it; for others it is just very flexible joints. Informal, everyday. Doctors have more careful names (hypermobile joints), but this is the word students will hear. It can be handy, or it can mean the joint is a bit less stable — both happen.