Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
infīnītīvus, a, um, adj. [infinitus], unlimited, indefinite: modus, or absol.: infī-nītīvus, i, m.; in gram., the infinitive, Mart. Cap. 3, § 310 sqq.; Isid. Orig. 1, 8; Diom. p. 331 P. al.