Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
tristĭmōnĭa, ae, f. [tristis], sadness, sorrowfulness, = tristitia (very rare): tristimoniā sollicitari, Auct. B. Afr. 10, 3: tristimoniam Ex animo deturbavit, Novat. ap. Fest. p. 372 M. (Com. Rel. v. 40 Rib.).
Collat. form tristĭmōnĭum, i, n.: esse in tristimonio, Petr. 63.