Green’s Dictionary of Slang

curtains n.²

1. (US gay) the foreskin.

1971		D. Rader Gov’t Inspected Meat 9: ‘He still got his curtains?’ meaning the foreskin, ‘or you let the Jew priests cut him up?’ [Simes:DLSS].
1972		‘D.L. Rinker’ City Boys 33: Nick watched the marvelous cock as it lifted, as the curtain that held the tender head was pushed back by the erection [Simes:DLSS].
1988		H. Max Gay (S)language.

2. the labia.

1959		L. Longmore Dispossessed 40: A mother: Ja, you Shoeshoe women!! You are terrible ‘pennylines’! You pull your labia minora in order to grab our husbands. One day I shall cut off those ‘long curtains’!
1982		Maledicta VI:1+2 (Summer/Winter) 132: Hottentot apron (elongation of labia majora, curtains).
2002		B. Healey ‘U4ME’ (poem) on Originality 🌐 (Not to be sexist) Making waves for the man in the boat / Opening the curtains and clearing your throat.

In phrases

draw the curtains (v.)

to fellate an uncircumcised penis.

1988

H. Max Gay (S)language.