a grin like a Cheshire cat

a very wide smile

Usage notes: The Cheshire cat is a character in Lewis Carroll's book Alice in Wonderland and is famous for its big smile.

I just presumed he'd got the job because he walked in here with a grin like a Cheshire cat.

Name	Pronounced	Usage
Matei	-	Romanian
JoŽE	YAW-zhe	Slovene
Celandine	SEL-ən-deen	English (Rare)
Flannery	FLAN-ə-ree	English (Rare)
Eugenia	e-oo-KHE-nyah (Spanish), e-uw-GEN-yah (Polish), yoo-JEE-nee-ə (English), yoo-JEEN-yə (English)	Italian, Spanish, Romanian, Polish, English, Ancient Greek (Latinized)
Lake	LAYK	English (Rare)