Wiktionary
shit-eating grins
n. (shit-eating grin English)
Usage examples of "shit-eating grins".
On the faces of her motley cavalry, shit-eating grins were suddenly wiped away, replaced by businesslike attention.
A couple years ago some Dutch researchers noticed that female stump-tail macaques (macaques are a type of monkey) displayed what we must frankly describe as shit-eating grins while engaging in, ahh, lesbian episodes.