"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?)

Also the place where I live - a small suburb of Dublin, Ireland. The name itself is an anglicisation - from "Domhnach Broc" - The Church of St. Broc in Irish. It came to mean fight or brawl for the reputation gained by the Donnybrook Fair which ran there from about 1204. I've never heard it used seriously, but I'm told it's common enough in Australia. See my writeup on the fair from more info.
