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common term for a person who shows lots of anger/grumpiness. When you perceive an angry person complaining or ranting, usually "Grumpy Cannon" is said to annoy/aggrivate them.

The term comes from a Megaman type game, as the thought is that if you beat the commonly angry person, you will inherit their grumpy cannon.
by Skylarke Van Holdzen February 04, 2005