Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-sĭbus, a, um, adj. [sibus, callidus sive acutus, Fest. p. 336], very knowing, very acute, Plaut. and Naev. ap. Fest. p. 217 Müll.; cf. Naev. ap. Varr. L. L. 7, § 107 Müll.