Dirty or well-thumbed bank notes. Denotes bank notes that have been stored in a lady's brassiere (i.e. near her numbis or breasts) for safe-keeping from pickpockets and muggers. They are usually produced from said brassiere when the owner needs to pay for something larger than a chappie (chewing gum), and are characterised by being dirty and somewhat damp.
The cashier handed me numbi-notes when she gave me my change, and I almost didn't want to accept it.