Lewis Short
mĕtăcismus (noun 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.
Charlton T. Lewis, Charles Short, A Latin Dictionary