Green’s Dictionary of Slang

1. the penis.

1686		Behn Lucky Chance V iv: Some rich Commodities, I am sure, and some fine Knick-knack will fall to thy share.
1896		Farmer Vocabula Amatoria (1966) 96: Don, m. The penis; ‘the knick-knack’.

2. the vagina.

1890–1904		Farmer & Henley Sl. and Its Analogues.
1896		Farmer Vocabula Amatoria (1966) 134: Forêt de bois-mort, f. The female pudendum; ‘the knick-knack’.

3. (gay) a small penis.

1972

B. Rodgers Queens’ Vernacular 185: small penis [...] knickknack.

4. see nick-nack n. (5)