Also known to the rest of the world as a boo.
I've also heard a rasberry (the type you do with your tongue or armpit, rather than the fruit)
referred to this way.
The phrase originates with the New York Yankees, and basically any time that something happens to the
team that they don't like, every local newspaper and sportscaster will leap at the excuse to use the phrase. You don't often see it in
casual, non-sports usage, but occasionally magazine writers may tie the phrase to a specific New York City
issue such as crime or traffic problems.
When the Yankees website was defaced with a pornographic picture on October 27, 2000, for example,
the headline
on APBNews was "Yankees Web Site Gets Obscene Bronx Cheer."
The Bronx Cheer is also a cocktail made with 2 oz of Apricot Brandy over ice and Rasberry Soda.
Whole rasberries and/or an orange peel can be used for garnish.
There has been some mention of the Bronx Cheer being a type of pale ale, also made primarily
with Rasberries.
It's also a song track on the album Boces by Mercury Rev.