A wrinkled, retracted penis of old dudes in the showers at a swimming pool mens changing room. Mini means small, billy means willy. These are comparable to party weiners at get togethers and other various social events.
"Kieron, its a minibilly fest today!"
Male genitalia, of such appearance that it is a close visual match to that of a Classic British Chipshop sausage™, or a german sausage.
Hmmm, I'll have a large portion of chips and a large wanger please, guv!