If someone has a “shit-eating” grin, what exactly does that mean?
Is it that the grin looks as if someone is eating shit? Because if someone’s eating shit, it would seem like they would be wincing or puking. Anything but grinning, unless they roll like that for fun.
Or is it that the “shit-eating grin” is meant to signal to others that they are the ones who should be eating shit, which would explain a bit the notion that a “shit-eating grin” is a form of smugness.