The quality or state of being chummy; friendliness, familiarity, or intimate companionship.
From 'chummy' (adjective) plus the suffix '-ness,' which creates abstract nouns from adjectives. This follows the pattern of 'happy' → 'happiness,' 'friendly' → 'friendliness.'
English uses '-ness' to turn almost any adjective into an abstract quality—'chumminess' lets us talk about friendship itself as a thing you can measure and feel.
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