Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
attĕnŭātĭo (adt-), ōnis, f. [attenuo], a diminishing, lessening (only in the two foll. exs.): attenuatio suspitionis, Auct. ad Her. 2, 2: verborum attenuatio, simplicity, id. ib. 4, 11; cf. attenuatus.