This is more specifically defined as accidental excretion in the standing position, where the faeces is squashed into flat pebbles by the buttocks. Assuming that it isnt too sticky and comes away whole, the biscuit may resemble a cookie, a jaffa cake or something bigger. Any identifiable food can prompt the addition to the name
sweetcorn bum biscuits are common
hwang cookies are usually found in seattle WA
faeces, poo, shit, turd.
I gotta go and bake a bum biscuit.