Bronx Cheer is used tongue in cheek and sarcastically because it is not a cheer. It is used to show disapproval. The term originated as a reference to the sound used by some baseball fanatics attending games at Yankee Stadium, which is located in the Bronx, New York City.
It used to be that making a Bronx cheer was to make a noise signifying silliness. It was made by sticking out the tongue between the lips and blowing to make a sound reminiscent of flatulence.