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The story

Learned borrowing from Late Latin irrefrāgābilis (‘irrefragable’) + English -able (suffix forming adjectives with the sense ‘able or fit to be done’). Irrefrāgābilis is derived from Latin ir- (variant of in- (prefix meaning ‘not’)) + refrāgārī + -bilis (suffix forming adjectives indicating a capacity or worth of being acted upon); while refrāgārī is the present active infinitive of refrāgor (‘to oppose, resist; to gainsay, thwart’) (further etymology uncertain, possibly from re- (prefix meaning ‘again’) + fragor (‘a breaking, shattering; a crash; din, uproar’) (from frangō (‘to break, shatter’), ultimately from Proto-Indo-European *bʰreg- (‘to break’))), formed as an antonym of suffrāgōr, the first-person singular present passive indicative of suffrāgō (‘to support; to vote for’).1uncertain The source flags this step as uncertain.