The phrase "bust a grumpy" is used to refer to anything that is excreted through the anus, including both gas and feces. To properly bust a grumpy, one must abide by the following rules:
-Claim that he or she is going to "bust a grumpy"
-Excrete something (be it solid, liquid, or gas) from the anus.
Tom:"Dude, I'm gonna bust a grumpy!"
Walter:"Oh, no! Wait!"
Walter:"Oh, man! Everybody watch out, Tom's bustin' grumpies!"