Other forms: grumps
A grouchy, complaining person is a grump. If you mope around all week during a rainy vacation with your family, don't be surprised if they call you a grump.
Before grump came to mean a cranky person, the phrase the grumps was used for "a fit of ill-humor." The word may stem from a now-obsolete adjective, grum, "morose or surly." If your neighbor is a grump, he won't be amused by the massive water balloon fight in your yard — but it still may be worth baking him some cookies as an apology for scaring his little dog.