Green’s Dictionary of Slang

sexual intercourse, often adulterous, enjoyed around lunchtime.

1683		Fifteen Real Comforts of Matrimony 92: [He] gave her a Nooning over her Husbands head upon the same Chest where he lay fast under lock and key.
1764		Low-life 70: Young married people [...] are retiring into their Chamber to take a Nooning, and afterwards a Nap.