Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† mĕtăcismus, i, m. [for mytacismus, = μυτακισμός, from μῦ], a frequent repetition of the letter M, Mart. Cap. 5, § 514; also the pronunciation of M at the close of a word before a word beginning with a vowel, metacism, Diom. p. 448 P.