Plural of grump; either multiple people who are grumpy or instances of grumpy complaining; can also be a verb meaning 'complains in a bad-tempered way.'
From grump, which may derive from onomatopoeia for growling (grrum-) combined with the jump/bump family of words. As a plural noun or third-person verb, it adds standard English grammatical markers.
The word 'grump' is probably onomatopoetic—it imitates the growling sound—which means 'grumps' lets us pluralize a sound imitation, showing how English treats sound-words as regular nouns!
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