shit-eating grin

Possibly from a contraction of "grinning like a ’possum eating shit",[1] later detached from meaning and persisting due to its vividness (cf. "there are many ways to skin a cat" or "happy as a clam").

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shit-eating grin (plural shit-eating grins)

1. (idiomatic, vulgar) A broad smile indicating self-awareness that may suggest self-satisfaction, smugness, discomfort, or embarrassment.
   • 2006, Ted Bell, Spy: A Thriller, →ISBN, page 29:

     He was at the wheel of his brand-new 1965 GTO, top down, wearing a super-sized shit-eating grin on his face. 

• The term is ambiguous and may indicate either a genuine broad smile (e.g. smug happiness) or a fake broad smile (e.g. trying to hide or get away with something).

• For more quotations using this term, see Citations:shit-eating grin.

• coprophagous grin (rare)
• fish-eating grin
• smirk
• simper

• Oxford English Dictionary, 2nd ed., 1989.