What is the origin of the phrase shit eating grin? How did it come to mean showing smugness or self-satisfaction of an individual's actions?
From the Urban Dictionary:
...these uses are documented in the Oxford English Dictionary no earlier than 1957
There have been similar expressions used quite far back:
In Book XXI of his History of Rome, Livy describes a Carthaginian sect of coprophages, the risus faecivorus, or shit-eating grin, being commonly displayed by its adherents.
Although, its origin is undetermined, they may have been incidents which caused the invention of this phrase. Below is an excerpt:
"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).