means A joint is the place where two things meet and connect — whether bones in the body, pieces of wood, or parts of any structure — and by extension a shared place (a bar or hangout) or, in slang, a hand-rolled cannabis cigarette.

from From Old French 'joint,' meaning a join or junction, itself the past participle of 'joindre,' to join — which traces back to Latin 'jungere,' to yoke or bind together (a cousin of 'junction,' 'conjugal,' and even the yoke that joins oxen). The anatomical sense came first; the meaning 'a shared establishment' is 19th-century American slang (originally with a seedy whiff, a place where shady types were 'jointly' present); the cannabis sense, also American slang, likely arrived in the early 20th century, perhaps from the idea of something passed around in such a place.