Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Ŏpĭter, ĭtĕris and ĭtris (cf. Prisc. p. 695 P.; gen. Opetris; cf. Borghes. Framm. de’ Fasti Cons. 1, p. 66), m. [ob-pater], a Roman prœnomen: Opiter est, cujus pater avo vivo mortuus est, Fest. p. 184 Müll.: Opiter Verginius, Liv. 2, 17; 2, 54.