Busking

Finally a great definition for busking (by Dr F.) It also contains a definition for a "back-busk":

It is even possible to experience a "Back-Busk". This is when two experienced Buskers are busking hard and a busk is answered by another busk. Upon realisation of such an occurance, it is both Buskers' duty to cease the conversation and buy each other a beer, as nobody likes to be caught busking. It displays a lack of skill and finesse.