I have never eaten shit. I suspect, however, that if I did, a grin would rank rather low on the list of reactions I would have.
How did the rather common, if somewhat vulgar, expression “shit eating grin” come into usage?
Yer pal,
Satan
Just a WAG but I use to have a dog that ate his own shit ( they do that you know) when I would chew his ass out for it, he would just set there and look at me with his tongue hanging out with what I could only describe as a shit eating grin.
“I’m the best there is Fats. Even if you beat me, I’m still the best.”
(Paul Newman in The Hustler)
My experience with this phrase is that it’s used when one is feeling smug or kind of embarassed. Kind of like getting caught doing something that shouldn’t be done and then not admitting it, even though all of the parties involved know the truth.
If I ate shit I wouldn’t want to admit it. So if someone caught me eating shit, I would have that same sort of grin on my face -
“C’mon! <grin> Do you really think I would eat shit? <bigger grin> I mean, seriously… Why would I eat shit? <grin some more>”
If you want to witness an actual shit-eating grin, watch the last scene of “Pink Flamingos.”
I think it is just another meaningless aphorism that just contains the word “shit”. It is similar to “I didn’t know whether to shit or go blind” - which, to me, never seemed like a hard choice to make.
I’m really suprised no one has posted a visual sample of a “shit eating grin”, especially since there’s one so readily available:-----> 
I’m surprised nobody has mentioned fel…
OOWWW
Stop throwing things at me…
Thats a shit licking pucker Farmer, not a shit eating grin.
Get your shits straight… 
I always assumed it was descriptive of the size and/or physical nature of the grin, not the emotive context or content of the grin. (Simply put, ya’d better have a big ol’ wide “grin” on yer face while eatin’ yer shit, iffen ya don’t wanna get it on yer lips, chin, or cheeks.) It’s not supposed (in my understanding) to infer that the grinner feels like, or looks like they feel like, they’re eating shit and enjoying it. It’s supposed to infer simply that their mouth has taken on similar characteristics to someone engaged in that pursuit, namely wide open-ness, with the lips pulled back somewhat from the teeth.
Now don’t you be talkin’ 'bout the grin on my 'moticon, here?
And that’s hear, not here, yunno?
I always assumed it was somehow related to an expression my father used/uses: “grin like a possum eating shit”. Anyone who knows anything about possums (jaw musculature, eating habits) that could say anything about that?