Green’s Dictionary of Slang

to tape the penis to the groin (usu. in the context of transvestism or transexuality), thus n. tucking.

1985		T.R. Houser Central Sl. 53: tuck [...] ‘Before I come out on the streets, I tuck, and the men love it.’.
1993–2003		D. Gulliver on Urban Dict. 🌐 tuck when a male does a tuck, he ‘tucks’ his penis and testes back through his legs which encompasses the look of a females genitals. does it look better tucked or untucked?
2021		H. Joyce Trans 85: There are [on a ‘gender-affirming’ Facegroup group] discussions of ‘tucking’ underpants for transgirls, which squash the genitals between the legs.
2023		S. Ayad When Kids Say They’re Trans 80: Tucking is a technique where the crotch bulge of the penis and testicles is hidden by using materials such as bandages or masking tape.