Answer:

'Cheshire grin' seems to be a phrase derived directly from the famous grinning Cheshire Cat in Alice in Wonderland. The cat was itself based on the well known phrase 'grin like a Cheshire cat'.

So a 'Cheshire grin' is an enormous grin, reminiscent of the Cheshire Cat's.

First answer by MisoSoup. Last edit by MisoSoup. Contributor trust: 104 [recommend contributor recommended]. Question popularity: 3 [recommend question].