The comparative form of grumpy, meaning more sulky, bad-tempered, or irritable than something else.
From grumpy (possibly influenced by grump, itself derived from onomatopoeia mimicking growling sounds) plus the comparative suffix -er. The -er suffix for comparatives is one of English's most ancient features, used since Old English.
The word 'grumpy' might come from grump, which may imitate the growling sound an irritable person makes—so the comparative 'grumpier' is literally 'more growl-like,' embedding onomatopoeia in grammar!
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