Green’s Dictionary of Slang

curtains n.²

1. (US gay) the foreskin.

1972		B. Rodgers Queens’ Vernacular 123: dangling foreskin of uncircumcised penis [...] curtains.
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.