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.
(1) Upon telling Elaine he knew a secret, Jerry donned a shit eating grin and refused to reveal it.
(2) Forced to admit defeat, the politician managed a shit eating grin as he called for and end to "bipartisan rhetoric".