Green’s Dictionary of Slang

you know what n.

1. [17C] the vagina.

2. [mid-17C–mid-18C] the penis.

3. [late 17C–early 18C; 1950s+] sexual intercourse.

4. [late 17C+] (also what you know, you know) anything the speaker does not wish to name specifically; often used euph.

5. [2000s] the buttocks.