Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
pernīcĭtas, ātis, f. [pernix], nimbleness, briskness, agility, swiftness, fleetness (class.): pernicitas deserit: consitus sum senectute, Plaut. Men. 5, 2, 4: adde pernicitatem et velocitatem, Cic. Tusc. 5, 15, 45; Caes. B. C. 3, 84: pedum pernicitas, Liv. 9, 16: homines saltatim currentes, vivacissimae pernicitatis, Gell. 9, 4, 9: equorum, Tac. H. 1, 79.