Sh!t eatting grin?

Who says this? And why?

Recently I have heard a couple of people use this expression and I can't imagine a stupider expression. Ok, ok, you got me. I can imagine it but I am hard pressed to do so. Alright already, get off my freaking back, I can come up with 2 or 3 stupider expressions inside a minute, but they would be made up, and not in use, like this stupid expression.

Why on earth, why on earth and in the heavens would anyone have a sh!t eating grin? They wouldn't. Poop is gross. Eating it is repugnant. Vile. It is disgusting. In fact, there is a scene in Anchor Man with Ron Burgundy where he is made to taste poop. It is obviously a low light for him He is disgusted by it and doesn't want to eat the poop. But then, when he realizes he is such a wretch that it doesn't matter, he eats the poop anyways. Does he grin during any part of this? No, he does not. Why? Because eating sh!t is not a grinning matter. So, in short, this is a stupid expression which should be stricken from vocabularies everywhere. Next time you need to explain a grin, think of something that actually makes people grin. Like cake. Or flirting. Or kittens. Or boobies. A boobie sucking grin? Appropriate. A sh!t eating grin? Non existent...