Green’s Dictionary of Slang

lay n.3

[OF lei, law, which itself is the root of the synon. law n.]

1. [mid-17C+] (UK Und.) any kind of criminal activity; usu. modified by a participle that denotes the speciality, e.g. chiving-lay under chiv n.1 ; clouting lay n.; crack lay under crack n.4 ; kid lay under kid n.1 ; maundering lay under maunder v.

2. [18C–1960s] any form of enterprise, business or occupation; often the terms or conditions of such a contract or job.

3. [mid-18C–19C] the life and practice of crime, as in the lay.

4. [early 19C] stolen goods.

5. [mid-19C–1940s] (US) one’s (hidden) intention or aim.

6. [mid-19C–1950s] (UK Und.) a place considered for robbing.

7. [mid-19C–1950s] (US) a state of affairs.

8. [1910s] (Aus.) a trick, a deception.

9. [1920s] an obsession, a subject.

In phrases

on the lay (also upon the lay)

[mid-18C–1910s] involved in some form of illegal activity.

pull one’s lay (v.)

[1950s] (US black) to do something, e.g. perform music.