Why is the ninth month called September?
late 13c., "a part of a body where two bones meet and move in contact with one another," from Old French joint "joint of the body" (12c.), from Latin iunctus "united, connected, associated," past participle of iungere "join" (see jugular). Related: Joints. Slang meaning of "place, building, establishment" (especially one where persons meet for shady activities) first recorded 1877, American English, from an earlier Anglo-Irish sense (1821), perhaps on the notion of a side-room, one "joined" to a main room. The original U.S. sense was especially of "an opium-smoking den."
Meaning "marijuana cigarette" (1938) is perhaps from notion of something often smoked in common, but there are other possibilities; earlier joint in drug slang meant "hypodermic outfit" (1935). Meaning "prison" is attested from 1953 but probably is older. Out of joint in the figurative sense is from early 15c. (literally, of bone displacement, late 14c.).
early 15c., "united," from Old French jointiz (adj.) and joint, literally "joined," past participle of joindre (see join (v.)).
A point of articulation between two or more bones, especially such a connection that allows motion.
beer joint, the big joint, call house, clip joint, creep-joint, eat high on the hog, grease joint, gyp joint, ham joint, hopjoint, juke joint, pull one's pud, put someone's nose out of joint, rib joint, schlock shop, square
[place senses fr early 1800s Anglo-Irish joint, ''low resort,'' perhaps from its being a nearby, joined room rather than a main room]