faecal matter in one of its more fluid states, such as diarrhoea
her sheets were covered in bumsauce
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.