A raucous expression of displeasure. The sarcastic reference is to how spectators at sporting events in New York City's borough of the Bronx—at Yankee Stadium, for a notable example—let players on visiting teams, and umpires too, know what was on their mind. The classic “Bronx cheer” sound was produced by compressing the lips and blowing, which replicated the sound of passing wind. That noise was earlier called a raspberry (or raspberry tart, the British rhyming slang for “fart”), from which the word “razz” came.