As far as I can tell, eating shit wouldn’t make me grin or be happy at all. :dubious:
(Almost put in GQ, but decided it was too Pointless and Mundane even for it)
I’ve wondered about that myself so of course in an attempt to find some type of answer I Googled the phrase and came up with this. Not an answer but still kinda funny.
Oh my…thank you for my daily hearty laugh today…
Here’s an interesting take on the rationale for the phrase.
I’ve always thought it meant that you were so happy about something that you’d still be smiling, even if you were eating shit.
- Peter Wiggen
I always thought it refered to dogs. Ever see a dog eating shit, that wasn’t happy about it?
I’ve never seen a dog eat shit. 
No dog I’ve ever had, anyway…
It is my pleasure to be of service. 
You’ve deprived your dogs of turd jerky!?!?
I should have mentioned that it’s cat shit they prefer. I don’t know why some dogs eat it and others don’t. But when they do, it’s like steamy brown crack. They just can’t get enough.
Well, I would imagine this guy had a pretty big grin before he was caught.
My father, never lacking for a colorful vulgarity, uses the phrase “Happier than a possum eating shit” to quantify extreme happiness. I’ve always assumed that that was where the phrase “shit eating grin” came from.
FYI (or TMI) my dog adores kitty crunchies (litterbox snacks) when he can get to the litterbox after the cat uses it and before I can remove them. Ick ick ick.
The old MIT student-assembled handbook, HoToGAMIT, used to have a glossary with SEG in it. The only definition given was “Ask an upperclassman. (Smile when you do)”. Mean, them HoToGAMIT editors.
HoToGAMIT was a semi-acronym for “How to Get Around MIT”, and it usede to be a lot of fun, filled with oddball quotes, strange pictures, and weird humor. I’ve seen recent editions at the MIT Coop, and they’re as sober and boring as a judge.
Kids, today.
It’s the grin I get just before I tell someone to ‘eat shit’.
Like a poker tell when your opponet just laid down four kings and you have four aces.
OMFG! That’s awesome! Caption: “You mean, I get to keep the ring!?”
I’ve always assumed a shit eating grin was a guilty but unrepentant one. Like, I eat shit voluntarily, because I like it…and I’m a little embarassed that you caught me."
I’ve heard “happy as a shit-eatin dog” a couple times, so that’s where I figured it came from. You know, that pleased but sure you’re about to be in trouble look they get.
I always figured that someone who would eat shit is very stupid.
Thus a “shit-eating grin” is a big, stupid grin. The grin of an idiot, if you will.
I had always assumed that it meant you were grinning as if your teeth were covered in shit – i.e., so wide your lips didn’t touch your teeth at all.
Damn…so many possible meanings. And they all sound plausable.
No, no, no. A shit-eating grin is the kind of obsequious, self-debasing smile you give to a boss or someone who has power over you, to indicate that whatever they say or do is fine with you, and that you know you are totally in their power. It comes from the idea of kissing ass - i.e. “yes, I kissed your ass so much my face is full of shit, and I enjoyed it, yes sir, can I have another?”
Hence the sense of a big, wide, fake smile.
Well, that’s my take on it, make of it what you will.