inmutesco

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.