grin is a facial expression characterized by turning up the corners of the mouth, often showing teeth.
Common usages of grin:
1. To smile broadly, especially in an unrestrained manner
2. To express amusement, pleasure, or smugness
3. Idiom: "grin and bear it" - to endure something unpleasant with good humor
4. Slang: "shit-eating grin" - an annoyingly smug or self-satisfied smile