Busking

Improvising. Often used when referring to running lights on the fly, with nothing pre-programmed. "I've never seen/heard this band and don't know the set-list, so I'm going to be busking the lighting." The failed Tommy Tune musical, Busker Alley, might explain the origins of the term, but the show closed before I could see it.

Tommy Tune played Charlie Baxter, a busker. "Busker" is British slang for a person who entertains people for money in public places (as by singing or dancing), usually while asking for money. As a busker usually has to improvise (given his/her surroundings, audience, etc.) this may be the origin of the term.