12 letters · 18 points in Scrabble
originLearned 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…
Etymology from Wiktionary, CC BY-SA 3.0