I think there's a pretty good explanation here
http://ask.metafilter.com/177958/Ety...hiteating-grin
Quote:
shit-eater noun coarse orig US
1. A contemptible or despicable person. 1937–.
2. A person or animal which eats faeces. 1968–. Hence shit-eating, adjective
1. shit-eating grin: (a) a broad grin expressing uncontrollable delight or (self-)satisfaction, esp. at having got away with something outrageous or improper. 1957–. (b) an ingratiating, embarrassed, or uncomfortable expression, esp. as exhibited by someone undergoing a humiliating experience; a forced smile. 1990–.
2. Contemptible, despicable. 1960–.
3. Of or engaging in the eating of faeces. 1970–.
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