"An uprorious brawl," according to my
paperback Merriam-Webster dictionary (1974 edition). Sometimes
capitalized.
I'm surprised Webster 1913 didn't pick this one up. Must be a post-1913 expression. My first exposure to the word was in the Star Trek Concordance, where "donnybrook" is used to describe the fight that interrupts Captain Kirk's game of fizzbin in "A Piece of the Action." (How's that for gratuitous hardlinking?)