Very friendly and close with someone, like best friends who share everything and always hang out together.
From the noun 'chum' (meaning close friend), which originated in British slang around the 1680s, possibly from 'chamber-mate.' The suffix '-y' creates the adjective form meaning 'resembling or characteristic of a chum.'
The word 'chum' might come from students sharing a chamber or room at Oxford, making roommates the original 'chums'—which is why being chummy is about sharing space and secrets like old college friends would.
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