My first husband, who came from a small town 100 miles north of my home town, used expressions I had never heard of such as "grinning like a goat chewing a quince", "grinning like a cat shitting razor blades" (very self explanatory!)

Reminds me of a "grinning like..." saying from South Mississippi: "Grinning like a mule eating briars." If you've ever seen a farm animal eating something with thorns or sharp, stiff stalks, they'll often chew with their mouths partially open in sort a sardonic or insincere-appearing "smile" to reduce irritation so the saying makes sense.

SC