Green’s Dictionary of Slang

1. the vagina.

a.1660		‘Holme’ Harleian Mss. 196.16v: [I have] a knack aboue my knee, long it is & deep it is, & in the midst a hole there is: for came a yong man & put in a thing two handfulls long.
a.1695		‘Nut Brown Lass’ in Catch Club Pt 1 5: She that has good Eyes has also good Thighs and a better knack.

2. the penis.

1890–1904

Farmer & Henley Sl. and Its Analogues.