In a completely opposite manner; with absolute contrast or opposition; in a way that represents the furthest possible distance apart.
From 'diametrical' + '-ly' (adverbial suffix). This creates the longest and most emphatic adverbial form of the diameter-derived family of opposition words.
When someone says two things are 'diametrically opposed,' they're borrowing geometry language to make a philosophical point—no middle ground exists, just like there's no middle ground on a diameter between its two endpoints. It's math used to win arguments.
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