Definition: a fight
Donnybrook was a town in Ireland (now a district of Dublin) which was home to the Donnybrook Fair. That event became legendary for alcohol consumption and subsequent fist fights.
Why I Looked It Up
Ronald Reagan wrote in his dairy about getting into a “donny brook” (he used the space, which I think is incorrect) with Tip O’Neill about Social Security policy, referring to a 45-minute yelling match they held during a White House meeting in 1982. Both Reagan and O’Neill were Irish, which might have encouraged the usage.