Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
trĭbūlis, is, m. [tribus], one of the same tribe with another.
trībŭlum (trīvŏl-), i, n. [tero], a threshing-sledge, consisting of a wooden platform studded underneath with sharp pieces of flint or with iron teeth, Varr. L. L. 5, § 21 Müll.; id. R. R. 1, 22, 1; 1, 52, 1; Plin. 18, 30, 72, § 298; Verg. G. 1, 164.
Collat. form trībŭla, ae, f., Col. 2, 20, 4; 1, 6, 23; 12, 52, 7; Vulg. 1 Par. 20, 3.
trĭbŭlus (trĭbŏl-), i, m., = τρίβολος, an instrument resting on three of its iron prongs, while a fourth projected upward, thrown on the ground to impede an enemy’s cavalry, a caltrop.