late 14c., mewet "silent, not speaking," from Old French muet "dumb, mute" (12c.), diminutive of mut, mo, from Latin mutus "silent, speechless, dumb," probably from imitative base *meue- (source also of Sanskrit mukah "dumb," Greek myein "to be shut," of the mouth). Form assimilated in 16c. to Latin mutus. The meaning "incapable of utterance, dumb" is by mid-15c.
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Harper, D. (n.d.). Etymology of mutism. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/mutism