Lewis Short
immūtesco (inm-), tŭi, 3
* V. inch. n. [in-mutesco], to become dumb or speechless (post-Aug.): immutescamus alioqui, si nihil dicendum videatur,Quint. 10, 3, 16: ruptis ore querelis,Stat. Th. 5, 542.
Charlton T. Lewis, Charles Short, A Latin Dictionary