fish-eating grin

A smug, self-satisfied, or overly pleased smile, especially one that (intentionally or unintentionally) irritates others. She's been walking around with that fish-eating grin on her face ever since she found out she got into Harvard. I really wanted to wipe that fish-eating grin off his face after he won the poker game. A: "What's up with the fish-eating grin?" B: "Oh, nothing. Just that Jenny—that's right, the most popular girl in school—said she'd go to prom with me."