Green’s Dictionary of Slang

(US black) the female genitals.

c.1720		Ballad n.p.: I heard the merry wag protest, The muff between her haunches, Resembled most a magpie’s nest, Between two lofty branches [F&H].
c.1800		Burns ‘Ken ye Na Our Lass, Bess?’ Merry Muses of Caledonia (1965) 180: Between her lily-white thies / She’s biggit a magpie’s nest.
1983		R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].