I've read many times recently that Michael Phelps is double-jointed. The term is totally misleading because it suggests that someone with this condition has twice as many joints as others have. Actually, the term double-jointed is connected to something called Hypermobility and means only that the joints can stretch farther than normal.
I wondered whether the word 'double' refers to the extent or degree of something in this context (not quantity), i.e. perhaps the joints can be stretched twice as much.
How come they call it double-jointed?