One of those sayings that just doesn't make sense. To me, anyway... but here goes:

I would best summarize this as a mischievous grin. The presence of such a grin on someone's face would indicate that this person is up to something - or, more likely, has just done something. Something devilish. Something one would want to "get away with." Something that would probably make you say "oh shit!" if you knew what it was... and you are probably just about to find out.

Now, let's pull this one apart. Shit-eating. That would be the consumption of excrement. While such a scatological delicacy may have the odd adherant here or there, it isn't particularly clever or mischievous. Many people find it distasteful in the extreme. Is it anything to grin about? Hmmm... well, I said it didn't make sense to me.

Then there's a friend of mine who has a different phrase of the same sort - that "just-fucked-the-neighbor's-cat grin." Go figure.