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.

To have to go number 2.
To poop.
To make naughty water.

having to take a crap.

having an orgasm, especially when one has an angry-looking o-face