Green’s Dictionary of Slang

lay n.2

[lay v.1 (1)]

1. [mid-17C; late 18C; 1920s+] a person with whom one has sexual intercourse, or a promiscuous woman; usu. qualified as a good lay, a bad lay, an easy lay etc.

2. [1920s+] (orig. US, also lay-to) an act of sexual intercourse.