Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Pennīnus (Penīnus or Poenī-nus; the latter orthog. on account of the false derivation from Poeni, because Hannibal marched over this mountain to Italy, Liv. 21, 38, 6 sqq.; Plin. 3, 17, 21, § 123), a, um, adj. [from the Celtic Pen or Penn, summit, peak], of or belonging to the Pennine Alps (between the Valais and Upper Italy, the highest point of which is the Great St. Bernard), Pennine: Alpes, Plin. 3, 17, 21, § 123; Tac. H. 1, 87; called also, juga, id. ib. 1, 61: mons, i. e. the Great St. Bernard, Sen. Ep. 31, 9; also, absol.: Penninus, Liv. 5, 35; 21, 38: iter, over the Great St. Bernard, Tac. H. 1, 70: VALLIS POENIN, the Valais, Inscr. Grut. 376, 6: DEO PENINO D. D., the local deity of the Pennine Alps, Inscr. Spon. Misc. Ant. p. 85, n. 30; called also, IVPPITER POENINVS, and simply, POENINVS, Inscr. Orell. 228 sq.