Green’s Dictionary of Slang

(gay) to reveal one’s sexual preferences by dropping broad hints; thus keep your hairpins up, maintain a ‘normal’ mask.

1964		Homosexuality & Citizenship in Florida 24: Glossary of Homosexual Terms [...] ‘hair pins’ or just ‘pins’: To drop hints to a person whose ways are unknown, to determine iof he is a homosexual or not .
1965		A. James America’s Homosexual Underground 142: Women who understand homosexuals are said to ‘know the score.’ They’re not supposed to be shocked by anything. So when they attend a party there’s no furtive whispering at the door. You don’t hear ‘Keep your hairpins up, dearies...There’s a “straight” inside.’.
1965		Guild Dict. Homosexual Terms 12: drop hairpins (v.): For a homosexual to hint about his inversion to another person in order to ascertain whether that person is also homosexual; to do this accidentally.
1970		J.P. Stanley ‘Homosexual Sl.’ in AS XLV:1/2 57: drop hairpins v Reveal one’s homosexuality by dropping hints.
1972		E. Newton Mother Camp 33: The process of sending out subtle cues or ‘feelers’ is called ‘dropping the hairpin.’ (This phrase is also used when one has made an outright admission of homosexuality).
1986–7		Maledicta IX 144: The effeminate ones (working girls) may drop a hairpin in conversation or otherwise advertise availability; this is called posting flyers.
1999		D. Lypchuk ‘A dirty little story’ in eye mag. 8 July 🌐 When she confronted him he suggested that she have a double peptide with his old saw, so she performed an orchidechtomy on him. After that she decided to drop her hairpins and show her true colours to the world.
2003		K. Cage Gayle 67/2: drop a hairpin v. give a subtle signal that one is gay [Western Cape, 1950s].