Most friendly, most familiar, or most intimate in relationship; superlative form of chummy.
From 'chummy' (friendly, like a chum) plus the superlative suffix '-est.' Represents the peak of friendliness on a spectrum from 'chum' → 'chummy' → 'chummier' → 'chummiest.'
The superlative 'chummiest' sounds almost silly in English—when bonds get deepest, we use the most reduced, informal language possible, like friendship shrinks our vocabulary to baby-talk.
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