The bum wee is consistency of water, approximate density of 1001 kg/m3 and can be coloured various different shades of brown (linked to what alcohol has been consumed the night before - for example - Very Dark Brown = Guiness)
It can cause some "irritation" of the bottom, but this will subside once the bum wee ceases.
Lewis: "Yeah, it's a bit bumwee though. Thats a nice bumwee you've got."
Tramp: "Is yer friend takin' the piss?"
"My GOD! It smells like bumwee in here! The smell is like getting hit in the face. WITH A SHIT!"