Meaning

noun a facial expression characterized by turning up the corners of the mouth; usually shows pleasure or amusement

verb to draw back the lips and reveal the teeth, in a smile, grimace, or snarl

originFrom Middle English grin, grinne, grine, from Old English grin (“snare, noose”), of uncertain origin, but probably from the merger of Proto-West Germanic *garni and *garnī (“intestine”), ultimately from Proto-Germanic *garniz, *garnō (“guts, intestine”). Related to English yarn.

Etymology from Wiktionary, CC BY-SA 3.0