Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
expurgātĭo, ōnis, f. [expurgo, II.], a justification, vindication, excuse (a Plautin. word): habui expurgationem, Plaut. Am. 3, 3, 10; id. Merc. 5, 3, 4 (Ritschl, expurigationem).