Lewis & Short

confirmātīvus, a, um, adj. [confirmo], serving for confirmation, confirmative: adverbia (profecto, scilicet, quippe, etc.), Prisc. pp. 1020 and 1056 P: conjunctio, Diom. 411 ib.; Prisc. p. 1032 ib.: verbum, Ambros. Abrah. 2, 11, 89.
Subst.: confirmātī-vum, i, n., an affirmation, affirmative: duo negativa confirmativum faciunt, Schol. Juv. 14, 127; 6, 457.
Adv.: confirmā-tīvē, Tert. adv. Marc. 4, 41; Prisc. p. 1149 P.