Green’s Dictionary of Slang

lap n.³

SE in slang uses

In phrases

night laps (n.) [SE laps of a pool, i.e. night exercise; but cf. lap v. (4)]

(US campus) sexual intercourse.

1984

Eble Campus Sl. Mar. 7: Sexual activity [...] Synonyms: Horizontal Twist and Shout, Night Laps.