Taste Like Chicken?

The other day I heard someone use the expression "shit-eating grin".  At first I didn't give any thought to it because it is a commonly used expression.  However, this morning it popped into my head as I was fixing my hair and it made me ponder.  Since most expressions are derived from common events at one point in time in the vast human history, when did the human species every eat shit and grin about it?  So, I did a little digging and this is what I have found.

Definition:

(1) n. A shit eating grin is a very wide and, to the outside observer, stupid looking grin, usually showing smugness, self-satisfaction, or inner humor. The term is most often seen in the expression "Wipe that shit eating grin off your face!", usually said by the aforementioned outside observer. This observer-based definition makes "shit eating grin" the negative counterpart to "You look like the cat who ate the canary." While the two expressions describe the same grin, they have very different connotations. This definition has nothing to do with the term "shit eater".

(2) n. Someone donning a forced smile in an uncomfortable, embarrassing, or compromising situation could be said to have a shit eating grin. Because an uncomfortable situation is much more closely related to "eating shit" than smugness, it is plausible to assume that this is the first definition, although far from the most common, and that the above definition is a result of drift from this one.

Both of these uses are documented in the Oxford English Dictionary no earlier than 1957.

As for the origin I found pretty much jack shit.  There was reference to an excerpt from Livy's History of Rome:  Book 21:

"1944 Jrnl. Nerv. & Mental Dis. XCIX. 959 Among demented patients in advanced stages of their illness,..it is not rare to see some of them grasp their own feces, chew them and eat them often with great pleasure and satisfaction (coprophagia).

So, there you have it.  Another bit of useless trivia to tuck away into the back of your brain so the next time someone says they had a shit eating grin on their face you can tell them to get there demented ass to a psychiatrist.