Green’s Dictionary of Slang

(US) to indulge in sexual activity at night, out-of-doors; thus moontanning n.

1947		Berrey & Van den Bark Amer. Thes. Sl. (Supplement).
2002		‘Katie’ ‘Taking a Bite Out of the Big Apple’ SqueakyLand 🌐 laverne: I bet you’re all wondering why I called you over here. squiggy: Well it better be good, we had to cut our moontanning short. lenny: Our dates had to be home by midnight.