The superlative form of grumpy, meaning the most sulky, bad-tempered, or irritable in a group.
From grumpy plus the superlative suffix -est, one of the oldest comparison mechanisms in English, used to mark the highest degree of an adjective.
The superlative 'grumpiest' shows English has two ancient comparison systems—adding -er/-est and using more/most—and we use both today, though grumpiest feels more natural than 'more grumpy' to most English ears.
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