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.