Thank you so much....I can explain, but only in English.
This isn't a saying or anything; it's just another instance where a noun is turned into an adjective.
A Jack-O-Lantern is a pumpkin with a scary face carved in it and a candle inside, for Halloween decoration. So a "Jack-O-Lantern grin" would be an evil, impish, or sly and toothy smile.
Sorry if this is a little pedantic. You never know if people have heard of your holiday traditions....