Green’s Dictionary of Slang

put n.2

[SE put, a card-game + pun on put, an act of thrusting or pushing and in 1. the vagina being ‘dark’; 2. SE two-handed, whole-hearted ]

In phrases

put in the dark (n.)

sexual intercourse.

Catologue of Contented Cuckolds [ballad] At midnight sometimes she returns with a Spark, / Say, I sometimes have found her at Put in the dark.