The grimace on her face when he asked her to the prom told him her answer was "no" before she said a word. A grimace is a facial expression that usually suggests disgust or pain, but sometimes comic exaggeration.
Picture someone wrinkling his nose, squeezing his eyes shut, and twisting his mouth and you'll have a pretty solid mental image of a grimace. It can be a verb, as in "the class grimaced at the teacher's suggestion of a pop quiz." Or it words as a noun. "The class gave a grimace when the teacher suggested a pop quiz." Its forerunner was the 17th century Spanish grimazo, meaning caricature, and grima, meaning fright.