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1650s, from French grimace, from Middle French grimache, from Old French grimuce "grotesque face, ugly mug," possibly from Frankish (cf. Old Saxon grima "face mask," Old English grima "mask, helmet"), from same Germanic root as grim (adj.). With pejorative suffix -azo (from Latin -aceus).
1762, from French grimacer, from grimace (see grimace (n.)). Related: Grimaced; grimacing.