The quality or state of being grum; moroseness, surliness, or ill-tempered gloominess.
From grum (morose) plus -ness suffix, which converts adjectives into abstract nouns describing the quality or state.
The -ness suffix is so productive in English that any adjective can become an abstract noun—'grumness' is instantly understandable even if rarely used, because English speakers intuitively know how this word-building machine works.
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