double-jointed Some individuals are referred to as being ‘double-joined’. In reality these individuals do not have ‘double’ joints, but have a greater than average range of joint mobility — the joints are hypermobile. This can have career advantages (e.g. for contortionists) but can sometimes result in painful joints even though there is no clinical evidence of joint disease (hypermobility syndrome).

William R. Ferrell

dou·ble-joint·ed • adj. (of a person) having unusually flexible joints, typically those of the fingers, arms, or legs.