Verb
grin like a Cheshire cat (third-person singular simple present grins like a Cheshire cat, present participle grinning like a Cheshire cat, simple past and past participle grinned like a Cheshire cat)
(simile) To smile broadly, especially in a self-satisfied way.
John ended the set with a beautiful serve, an ace, and could not help grinning like a Cheshire cat.
Synonym: smile from ear to ear