To grin like a cheshire cat
About 500 years after the Norman conquest of England, the county of Cheshire became an independent political unit with its own tax collectors, courts, and parliament, and very little law enforcement. Because of this lack of punishment, the great Delamere Forest became notorious as a hideout for highwaymen and every other type of gangster.
During the reign of Richard Ⅲ, a Mr. Caterling was appointed to the position of Forest Warden of that criminal district. Caterling took his job so seriously that he quickly cleaned out all of the criminals and is said to have had more than 100 lawbreakers hanged.
A dedicated man, Caterling took great pride in his record and appeared at public functions with a wide grin. His grin was in fact so wide that it was almost comical, and it is said that people flocked to these public gatherings just to look at Caterling's grin. Soon an expression started making the rounds that was not entirely kind to Caterline: anyone who grinned a very wide grin was said to "grin like a Cheshire Caterling."
Later the expression was simplified to "grin like a Cheshire cat".