Green’s Dictionary of Slang

1. (US) a condom [SE safe adj.].

1870		in M.W. Hill Sisters’ Keepers 236: [advert] French Imported Male Safes—A Perfect Shield Against Disease or Conception, Made of both Skin & India Rubber.
1885		Nat. Police Gaz. 26 Sept. n.p.: [advert] Genuine French photographs [...] Red-hot, in sets of three [...] Rubber safes for gentlemen $5 per dozen .
1938–41		in G. Legman Limerick (1953) 288: With a safe for protection / He’d get an erection, / And then balance himself on his prick.
1946		in G. Legman Limerick (1953) 231: A fellow who slept with a whore / Used a safe, but his pecker got sore.
1957		G. Metalious Peyton Place (1959) 207: ‘Smart guy,’ she was hissing at him [...] ‘So smart he doesn’t even know to wear a safe.’.
1961		Realist June n.p.: A man slimeys up the counter. ‘Three Rameses please [...] Don’t tell me they have a tax on safes now?’ ‘No, the price has gone up — $2.50 for three.’.
1970		(con. 1950s) H. Junker ‘The Fifties’ in Eisen Age of Rock 2 (1970) 102: A circular bulge (rubber, safe, skin) [...] etched itself into his wallet.
1989		J. Morton Lowspeak.
2003		G. Carlin GeorgeCarlin.com 🌐 Condom: safe.
2012		G. Pelecanos (con. 1972) What It Was 98: That boy’s gonna shoot while she’s fittin the safe on his pecker.

2. (US prison) as a container [the image of the SE safe, into which things can be placed for security].