Superlative form of grum; the most morose, surly, or ill-tempered.
Superlative adjective formed from grum by adding -est suffix (standard English superlative formation).
'Grummest' shows how English speakers can coin or resurrect archaic adjectives and run them through modern grammatical patterns—even if grum is obscure, any English speaker instinctively understands 'grummest' means 'most grum.'
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