The first publication to mention this eccentric image was the work of satirist Peter Pindar (the pseudonym of John Wolcot), who wrote A Pair of Lyric Epistles (1795) containing the line:
Lo, like a Cheshire cat our court will grin!
Seventy years later William Makepeace Thackeray in The Newcomes also referred to the infamous grinning cat:
Mr Newcome says to Mr Pendennis in his droll humorous way, ‘That woman grins like a Cheshire cat.’
Then in 1865 Lewis Carroll’s Alice’s Adventures in Wonderland brought the Cheshire cat to centre stage. Carroll didn’t invent the image, but he certainly brought it wide international awareness and usage. There are various theories as to why Cheshire cats are described as grinning. Round Cheshire cheeses with semi-circular ‘smiles’ impressed on them is one possibility. But when Alice asks the Duchess why the cat has a grin, she replies, ‘It’s a Cheshire cat, and that’s why.’
With that we have to be satisfied.