Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
nĕgātīvus, a, um, adj. [nego], that denies, negative (post-class. for negans, privans, etc.): negativa actio, Gai. Inst. 4, 3: particula, App. Dogm. Plat. 3, p. 32, 36: verba, Dig. 50, 16, 237; opp. confirmativus, Schol. Juv. 6, 457; 14, 127.
Hence, adv.: nĕgātīvē, negatively (opp. affirmative), Cassiod. Dial. p. 548