Green’s Dictionary of Slang

drop (one’s) hairpins v.

also drop the hairpin
[SE hairpins are seen as a quintessentially feminine possession]

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

[US]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 .
[US]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.’.
[US]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.
[US]J.P. Stanley ‘Homosexual Sl.’ in AS XLV:1/2 57: drop hairpins v Reveal one’s homosexuality by dropping hints.
[US]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).
[US]Maledicta IX 144: The effeminate ones (working girls) may drop a hairpin in conversation or otherwise advertise availability; this is called posting flyers.
[US]D. Lypchuk ‘A dirty little story’ in eye mag. 8 July [Internet] 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.
[SA]K. Cage Gayle 67/2: drop a hairpin v. give a subtle signal that one is gay [Western Cape, 1950s].