The slightly moist feeling between one's buttocks, specially around the anus generally felt on a hot day, after a day of heavy bum burping or after exercise. Bumsauce is not mearly sweat but actual secretion from the bum and is normally quite smelly. Also called sebum.
Note: this is NOT the same as the squits.
My goodness Mavis, you've got quite a beefy arse this evening. Best you wipe off the bumsauce with your knickers before I go down on you.