It's a game. When you burp, you or your friends say "baganga". Then you have to put your thumb to your forehead and wiggle your fingers, pointed upward. If you don't, you'll get smacked on the forehead.
Exclamation, used in place of various four letter words. Conceived as an alternative expletive to use around children. Can pose as virtually any type of expletive: adjective, adverb, noun, verb.