Same word as unassailable:

absolute, ascertained, authoritative, clear, conclusive, confirmable, definite, demonstrable, destined, determined, establishable, evident, firm, fixed, genuine, guaranteed, hard, having down pat, in the bag, inarguable, incontestable, incontrovertible, indisputable, indubitable, infallible, irrefutable, known, on ice, plain, positive, predestined, provable, real, reliable, safe, salted away, set, sound, supreme, sure, sure thing, true, trustworthy, unambiguous, undeniable, undisputable, undoubted, unequivocal, unerring, unmistakable, unquestionable, verifiable