A)The grin on someone's face when they are trying not to smile but cant stop so they look like a total shithead.
called a shit eating grin because:
if you ate a piece of shit and you enjoyed it, you would naturally smile, but also try to hide that you enjoyed it at the same time, thus resulting in a "shit eating" grin.
"Wipe that shit eating grin off your face."
After John beat his friend Jim on a test, a shit eating grin came over him.
person 1 "Look at the size of that guys shit eating grin!"
person 2 "Man he looks retarded."