grin 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.