Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Trans-alpīnus, a, um, adj., that is or lies beyond the Alps, Transalpine: Gallia, Caes. B. G. 7, 1; 7, 6; Cic. Mur. 41, 89: cognatio materna Transalpini sanguinis, id. Red. in Sen. 7, 15: nationes, id. Fam. 9, 15, 2: bella, id. Off. 2, 8, 28.
In plur. subst.: Transalpīni, ōrum, m., nations beyond the Alps, Transalpine nations: legio una ex Transalpinis conscripta, Suet. Caes. 24.