From Latin 'junctus' (joined), from 'jungere' — literally 'a place that has been joined,' where two parts connect.
A point at which parts are joined; a structure in the body connecting bones; shared by or belonging to two or more people.
From Old French joint, past participle of joindre (to join), from Latin junctus (joined), past participle of jungere (to join, to yoke), from PIE *yewg- (to join, to yoke). A joint is literally a place that has been joined — the point where two things meet and connect. The anatomical sense (the joint of a bone) and the carpentry sense (a wood joint) both
The slang 'joint' meaning a place (as in 'a jazz joint') developed in nineteenth-century American English, probably from the sense of a joint as a place where things come together — a meeting place. The further slang of 'joint' for a marijuana cigarette (1930s) may come from the idea of a rolled, jointed thing, or from 'joint' as a disreputable place where drugs were used. The prison slang 'the joint' (1950s) extended the disreputable-place