Green’s Dictionary of Slang

1. (US) complete, unassailable; also as n. that which is guaranteed, e.g. a racehorse.

1894		Mirror of Life 9 June 2/3: We got a dispatch, which said play all in on an ‘air-tight’.
1896		F. Hutcheson Barkeep Stories 83: De pup cracks [...] ‘Well, wot d’ you t’ink you’ll do wid de mare to-day?’ kickin’ me in de shins at de same time. ‘I don’t see how I kin lose,’ says I. ‘I t’ink it’s an airtight’ .
1899–1900		C.L. Cullen Tales of the Ex-Tanks 194: I concluded to have one more whack at the ring on an air-tight good thing.
1925		D. Hammett ‘Dead Yellow Women’ Story Omnibus (1966) 157: It’s got to be air-tight on the other end [...] if we’re going to catch our fish.
1951		D. Dressler Parole Chief 27: Eugene [...] decided to make his report to me at once – possibly to establish a near-airtight alibi.
1953		W.P. McGivern Big Heat 95: Six of the men had air-tight alibis.
1974		Eble Campus Sl. Mar. 1: airtight loan – a safe risk.
1983		N. Heard House of Slammers 90: Edward T. Pagent the narcotic agent / Has built up an air-tight case.

2. (US) attractive.

1922

Appleton Post-Crescent (WI) 28 Apr. 17/4: Flapper Dictionary air tight – Very attractive.

3. (US Und.) of a town or city, absolutely safe for criminal activities since the authorities have been fully and comprehensively paid off.

1940

D. Maurer Big Con 226: If a city provides complete [...] protection for one or more favoured con mobs, it is known as ‘airtight’.

4. extremely mean.

1979

‘Iceberg Slim’ Airtight Willie and Me 15: Willie was air tight all right. He wouldn’t even spring for the motel fin.

5. (US gay) with every body orifice filled.

1972

B. Rodgers Queens’ Vernacular 20: airtight said when all the body orifices (as opposed to the cock) are in sexual use ‘At the gang-bang, the blond one was sure airtight: some dude was even dorkin’ him in the ear.’.