Green’s Dictionary of Slang

1. the female genital area.

1611		L. Barry Ram-Alley III i: And is not her halfe-moone mine?
1890–1904		Farmer & Henley Sl. and Its Analogues.
1896		Farmer Vocabula Amatoria (1966) 191: Mosle, m. The female pudendum; ‘the half-moon’.

2. a wig; also as alternative to head.

c.1642

T. Killigrew The Parson’s Wedding (1664) III v: Score a quart of sack in the half-moon?

3. a patch of sweat under the arm.

1978

R. Price Ladies’ Man (1985) 28: His chest bursting out of his floral shirt, half-moons under the arms.

4. in drug uses.