1.) A milder form of swamp ass, sometimes caused by a trickle of backsweat down the crack, usually on a female, can be very sexy and make you want to do butt.
2.) Early-morning swamp ass
3.) A classier term for swamp ass
Incidentally it is easier to encode "dew butt" into common speech to signify that your ass is sweaty to knowledgeable listeners.
Gent 1: Dear me, but I'm afraid there is a wetness in the crack of my bum!
Gent 2: Ah yes, with the heat and humidity being as it is, I fear I too have the dew butt.
When girls get dew butt that means no lube necessary.