1650s, from French grimace (15c.) "grotesque face, ugly mug," possibly from Frankish or another Germanic source (compare Old Saxon grima "face mask," Old English grima "mask, helmet"), from the same root as grim (adj.). With pejorative suffix -azo (from Latin -aceus).
1707, from French grimacer, from grimace "grotesque face" (see grimace (n.)). Related: Grimaced; grimacing.
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