In an equable manner; with calmness, evenness of temper, or uniformity.
From equable + -ly (in the manner of). Equable comes from Latin aequabilis, and -ly is a Germanic adverbial suffix that converts adjectives into adverbs.
You rarely see 'equably' in modern writing—it's one of those perfectly logical adverbs that feels oddly formal and out of place in contemporary speech, which shows how English naturalness doesn't always follow morphological rules.
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