Although similar, this slang has nothing to do with the term “shit eater”.
Shit-eating grin refers to a big smile on someone’s face, often showing self-satisfaction and a hint of some kind of internal joke going on. It is a smug, suspicious smile.
The villain in action movies will often have this facial expression when they have finally captured the superhero, before spoiling their evil plan in details.
The term is similar to the saying “looking like the cat who ate the canary”, although Shit-eating grin has a much stronger negative connotation to it.
As documented by the Oxford English Dictionary, the slang was first used in literature in 1957.
There are two theories of where the saying originates from:
- One is lowering the person with the smug grin down to the level of an animal, stating that they have no sense of social norms.
- The other is to refer to the the freaky grins of coprophages (people who eat fecal matter) in ancient Rome.
The term is semi-frequently used in English speaking countries as of today, but because of the vulgarity of the term, mostly by younger generations.
You would tell a person to “wipe that Shit-eating grin” off their face, if they did something questionable and you are getting uncomfortable.