grin like a Cheshire cat

 

have a broad fixed smile on your face.
    ✔ The Cheshire cat with its broad grin is best known for its appearance (and disappearance) in Lewis Carroll's Alice's Adventures in Wonderland (1865), but the expression, which is of uncertain origin, is recorded from the first half of the 19th century.  

Cheshire ˈ/cheshər/ 

There he stood, smiling like a Cheshire cat, waiting for his weekly pay.