grin like a Cheshire cat

To smile smugly or mischievously. The term was popularized by the character in Lewis Carroll's Alice's Adventures in Wonderland. I knew he had a prank planned for April Fools' Day when he arrived at work grinning like a Cheshire cat.

Smile broadly, especially in a self-satisfied way. For example, John ended the set with a beautiful serve, an ace, and couldn't help grinning like a Cheshire cat . The ultimate origin of this expression, appearing in print since the late 1700s, is disputed, but its most famous exponent was Lewis Carroll, in whose Alice's Adventures in Wonderland the grinning cat gradually vanished from view, with its grin the last part to vanish.

To smile broadly. The origin of this expression, which was well known in the eighteenth century, has been lost. Most explanations involve the traditional Cheshire County cheese, sold in the shape of a wheel and, perhaps, once so molded that it resembled a cat’s grinning face. Another theory holds that it refers to a Cheshire forest ranger named Caterling who frightened off poachers with his fierce grin. At any rate, the term has been appearing in print since the late 1700s in the works of such writers as William Thackeray and Charles Lamb, as well as in the work of its most famous exponent, Lewis Carroll.