The quality or state of being distempered; irritability or emotional imbalance.
From distempered + -ness (suffix forming nouns from adjectives). The -ness suffix is Old English in origin and creates abstract nouns describing qualities or states.
The suffix -ness is so common we barely notice it, but it's been turning adjectives into nouns for over a thousand years—it's how English lets us talk about abstract qualities like 'sadness,' 'happiness,' and in this case, emotional disorder.
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