Green’s Dictionary of Slang

garage n.

1. (US) a restaurant.

1917

R. Lardner ‘Three Kings and a Pair’ in Gullible’s Travels 76: When we got outside I left Bess and Bishop lead the way, hopin’ they’d head to’rds a steak garage.

2. the vagina.

1963

in T.I. Rubin Sweet Daddy 54: Hardly wait to shove it home. Wham into the old garage.

In phrases

garage door is open

(US) a phr. used to warn a man that his fly is open.

1972		B. Rodgers Queens’ Vernacular 93: garage door the pants fly ‘He opens his garage door, honey, an’ a big black Cadillac comes out, honkin’ for orders.’.
2002		‘Adult Joke Page’ on Reno’s Laugh 🌐 Noticing that her boss’s fly was open, the embarrassed secretary told him, ‘Your garage door is open.’.

In exclamations

shut your garage!

(US teen) be quiet!

1933

Phila. Daily Bulletin 1 Feb. 6: Shut your garage — close your mouth.