Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ad-verbĭum, ii, n. [verbum], in gram., an adverb, ἐπίρρημα; acc. to Priscian’s expl.: pars orationis indeclinabilis, cujus significatio verbis adicitur, p. 1003 P.; Quint. 1, 5, 48; 50; 9, 3, 53; 11, 3, 87 al.