In a manner relating to or along a diameter; in a way that is completely opposite or in absolute contrast.
From 'diametral' + '-ly' (adverbial suffix). The adverb form evolved naturally from the adjective to describe actions or states that embody complete opposition or measurement through a center.
Politicians love saying their opponents are 'diametrally opposed' to them—it's a dramatic way of saying there's literally nothing they agree on, with no middle ground between them. It's geometry language weaponized for argument.
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