Green’s Dictionary of Slang

1. (US) a place to sleep, a bed.

1919

C.E. Piesbergen Overseas with an Aero Squadron 58: ‘Me dogs are hurtin’, I think I’ll fix de lay,’ which means in English, ‘My feet ache, I think I’ll go to bed.’.

2. (US drugs) the act of lying down and smoking opium.

1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 123/2: lay. 1. A place to lie down and smoke opium. 2. The act of smoking.
1949		Monteleone Criminal Sl. (rev. edn).
1959		J.E. Schmidt Narcotics Lingo and Lore.

3. (US drugs) a session in an opium den.

1936

D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 123/2: lay. […] 3. The privilege of indulging one’s self in a den, as ‘How’s chances for a lay?’.

4. (US drugs) a portion of any type of drug.

1938

D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 2 in AS XIII:3 186/2: lay. 4. A ration of narcotics, not restricted to opium.