Where does the expression, shit eating grin, come from?

LOL! Way down here in the deep South!

I'm going to guess that this expression came about because of the way your mouth/face looks when you say "eat shit." The teeth are bared and very much like a big smile. Furthermore, one can EASILY say the "eat shit" phrase to someone with a big smile on their face, covering the fact (from afar at least) that you are insulting them.

In the days gone by, there were many horse drawn wagons in the towns. The stray dogs of the town, would follow the wagons and would eat the freshly deposited horse balls. Due to the taste, the dogs would skin their lips back in what appeared to be a grin.
Hense the phrase "shit earing grin".

Share your answer...

A shit eating grin is the kind of grin a person may front when caught lying while trying to play off the truth.

For example: Bobby Joe goes out to the pasture one evening to call Billy Bob for supper and catches Billy Bob getting excited while performing coprophilia (shit eating fetish) in a fresh pile of sheep shit and confronts him about it.

Bobby Joe: "Billy Bob? What the hell are you doing?"

Billy Bob: Stands up and gives a sheepish (pun intended) 'shit eating' grin, "Just investigatin' to check if the sheep got worms" he replies.

Needless to say, Billy got caught eating shit and played it off smooth with a shit eating grin to front the lie.