Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-tĕnĕo, ŭi, 2, v. a., to hold constantly, keep hold of, hold fast (late Lat.): utinam (me) pertinuisset, Hier. Ep. 66, n. 13.
per-tĕnŭis, e, adj., very thin, very small or fine.