Superlative form of grouchy: most bad-tempered, irritable, or inclined to complain and grumble.
From grouchy plus the superlative suffix -est. Follows the standard pattern for forming the most extreme form of an adjective.
When English speakers want to emphasize someone's grouchiness, we add more syllables and sounds—'grouchy, grouchier, grouchiest'—as if the word itself gets crankier!
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