Wiktionary
n. (context idiomatic vulgar English) A broad smile indicating self-awareness that may suggest self-satisfaction, smugness, discomfort, or embarrassment.
Usage examples of "shit-eating grin".
On the first day of my sophomore year, Joe McKennedy walked up to me through the crowded hall with his ConTact strip pasted on his forehead and a big shit-eating grin pasted on his mouth.
Then he looked down at Arnie again and gave him a large, kindly smile that was far too much like the previous shit-eating grin for me.
Some of those hot, muggy July nights Hank would lie awake in bed, head aching, and imagine Pits sitting in a nice warm farmhouse, shirt and shoes off, stinking to high heaven and grinning a great big shit-eating grin as he practiced cheating and dreamed of the suckers he would fleece when he got back home.
There were people who went into full-blown post-traumatic heeb-jeebs after a couple of sessions on FATSS, but Rydell always came out of it with this shit-eating grin.
Maybe he was just textbook-smart and life-stupid, `cause the shit-eating grin broke free on his face.
And Schneider always comes back with this big shit-eating grin, so I figure, you know.
My teeth hurt because I was paddling through the cold rain with them bared in a huge shit-eating grin.
The man had the look of a sun-bleached cowboy, from his shit-eating grin to his worn-heeled boots.