Green’s Dictionary of Slang

shit-eating grin n.

also cat-eating-shit grin, crud-eating grin, shit-eating smile, shit-eater grin
[shit-eating adj.]

(orig. US) a smug, self-satisfied smile.

[US]E. Hunter ‘The Jungle Kids’ in Jungle Kids (1967) 119: He was standing up there with a big crud-eating grin on his face.
[US]in E. Cray Erotic Muse (1992) 406: And it’s with great pride that he tells this story / With a shit-eating grin on his face.
[UK](con. WWII) G. Sire Deathmakers 196: Halvorson gives her this shit-eating grin.
[US]H. Selby Jr Last Exit to Brooklyn (1966) 62: Spook went puttin off with a shiteatin grin on his face.
[US]E. Thompson Garden of Sand (1981) 315: That Mississippi cat-eatin-shit-grin.
[US]T. Thackrey Thief 191: She still hadn’t lost that shit-eating grin.
[US]C. Loken Come Monday Morning 148: Andy smiled his shit-eatin’ smile leaned in patted Russ on the shoulder.
[US]E. Torres After Hours 186: I could see the shit-eatin’ grin on Dave’s face.
[US]W. Wharton Midnight Clear 154: You have that Buddha shit-eating smile all the time.
[US]S. King Christine 9: Then a large resplendent shit-eating grin appeared.
[SA]R. Malan My Traitor’s Heart (1991) 85: I gave him my shit-eating grin, saying brother this, brother that.
[US]Rebennack & Rummel Under A Hoodoo Moon 205: He got up, a big shit-eating grin on his face.
[UK](con. 1960s) A. Frewin London Blues 277: I hand them [i.e. photographs] back to Desmond, who is still sporting this shit-eating grin.
[UK]M. Manning Get Your Cock Out 16: Mince [...] stuffed his hand up the parson’s nose, a big old shit-eating grin cracking his chops.
[US](con. 1964–8) J. Ellroy Cold Six Thousand 445: Hombres todos — men with shit-eater grins.
[US]Chicago Trib. 17 Jan. 7A-4/3: His trade-mark crud-eating grin.
[US]J. Ellroy ‘Hollywood Fuck Pad’ in Destination: Morgue! (2004) 224: Big, burly, curly-haired, gap-toothed. Monstrous shit-eating grin.
[SA]A. Lovejoy Acid Alex 91: Titch strolled out of the bedroom. [...] He had a big shit-eating grin on his face and asked if anybody wanted to go again for seconds?
[Aus]L. Redhead Cherry Pie [ebook] A lean man [...] wearing an open necked bodyshirt, a gold chain, and a shit eating grin.
[US]A. Steinberg Running the Books 78: [I] saw a group of inmates standing behind me. big shit-eating grins on their faces.
[US]D. Winslow The Force [ebook] He’s got a big shit-eating grin on his face.