The quality, state, or condition of being grumpy; chronic bad temper or sullenness.
From grumpy with the abstract noun suffix -ness, one of the most productive suffixes in English for creating nouns from adjectives. -ness comes from Old English and creates nouns describing qualities or states.
The suffix -ness has been used to create abstract nouns for over 1,000 years in English—it's so useful that we add it to almost any adjective, which is why 'grumpiness' feels instantly natural even if you've never seen it before.
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