Green’s Dictionary of Slang

of a woman, becoming damp with sexual arousal.

[1732		Delightful Adventures of Honest John Cole 22: Let his Wife be full brisk, / Bound, caper, and frisk, / Till she foams at the Thing that’s below, Sir].
2000		G. Young Corrie.net 27 Nov. 🌐 Ken should be concentrating on Deirdre. Surely he must have picked up on how absent and distant she’s become lately, particularly when Dev is around or mentioned? Dev has just come back from a trade fair, with a box of Belgian truffles for Drear as a gift, and now poor old Drear is fair foaming at the gash!