Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-sĕvērus, a, um, adj., very strict: imperium, Tac. A. 15, 48.
Adv.: persĕ-vērē, very strictly: inquirere, Plin. Ep. 9, 5, 1.