An illness / infliction often found to affect 18 - 30 yr olds the morning after a big night out.
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.
The beer that bums buy, with their hodge-podge of coins and wrinkled, depressing bills. Usually obscure, depressing brands that make up in alcohol percentage what they lack in flavour, or anything out of the singles cooler.
"Why'd you buy Extra Old Stock, brah? Man, that shite is bumbeer that makes my tongue tingle."