Green’s Dictionary of Slang

lay up v.

1. [mid-19C+] (also lay to) to rest, to relax.

2. [late 19C] (US) to die.

3. [1920s] to live, to be in a place.

4. [1920s+] (US, also lie up) to hide, to take refuge.

5. see lay back v.

In phrases

lay up in (v.)

[2000s] (US black) of a man, to penetrate a woman.