Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
* trĭ-portentum, i, n. [ter], an extraordinary omen or portent: deūm triportenta. Pac. ap. Varr. L. L. 7, § 59 Müll. (Trag. Rel. v. 381 Rib.).