A grin which someone crazy enough to be eating shit and be deliriously happy about it would have on their face.

having been invited to eat shit, one declines ... with a smile ...

Sometimes shit-eating grin.

(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, embarassing, 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.

The smirk on one's face that makes you want to punch it.

See: Obama

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.

1. A huge smile that shit eaters can't stand(aka people who take themselves too seriously) When a serious butthurt occurs followed by a SEG said "victim" often flees to Urban Dictionary to bitch and moan about SEG's or someone wearing one. These whiners are often on the wrong end of the stick(no pun intended) and are too dull to grasp that their bitching only intensifies the "shittiness" of the grin. Naturally humiliation of the "victim" is in direct correlation to the "shittiness" of the grin.

In any situation it's best to be the person wearing a shit eating grin as opposed to the person who just ate shit. The person wearing the shit eating grin possesses both self-control and inner contentment concerning present circumstances. (possibly but not always at someone's expense) .

Many women love a shit eating grin, as it expresses the mystery of inner laughter as well as a completely unabashed authenticity regarding another person. All without blabbing on about some bullshit she probably doesn't care about.

A shit eating grin is one of the most powerful/sexy smiles in the world when expressed correctly.

facial expression of those caught lying while not accepting resposibility

A stupid grin that makes you look so clueless that you enjoy eating shit on a daily basis without realising you're a complete putz.