Spilled coffee in the lap area, especially common when using the crotch as a cupholder on the morning commute. Fresh, it's a wet stain. Dry, it's a brown stain that can last all day. Occasional whiffs of coffee will drift upward through the day, especially during strenuous activity.
The bump in the road caused a splash, and the warm wetness indicated another case of coffee crotch
A girl with dark stains on the sides of her pussy. It has the appearance that she has coffee stains between her legs. The discoloration is enough to scare even the horniest of men.
That chick has a bad case of coffee crotch!
Joe: What is that ugly stain on the sides of her pussy?
Sam: Oh, she has a bad case of Coffee Crotch!