Comparative form of grum; more morose, surly, or ill-tempered.
Comparative adjective formed from grum by adding -er suffix (standard English comparative formation for short adjectives).
Irregular comparatives in English (like 'good/better' or 'bad/worse') are holdovers from Old English, but most short adjectives use -er/-est—'grummer/grummest' follows this predictable pattern that dominates modern English.
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