1: a facial expression usually of disgust, disapproval, or pain
Tiger made a painful grimace as the nurse manipulated his leg.
As a noun, grimace goes back to 1651 (it did not appear as a verb in English until 1732). We got it from the Middle French grimache. The French word is said to have a Germanic origin, which is funny since it all comes back to the Old English word grīma, meaning mask.