Green’s Dictionary of Slang

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

1950		R. Starnes Another Mug for the Bier 25: ‘Whyn’t you wipe that garbage-eating grin off your face?’ I asked.
1955		E. Hunter ‘The Jungle Kids’ in Jungle Kids (1967) 119: He was standing up there with a big crud-eating grin on his face.
1959		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.
1960		(con. WWII) G. Sire Deathmakers 196: Halvorson gives her this shit-eating grin.
1964		H. Selby Jr Last Exit to Brooklyn 87: Spook went puttin off with a shiteatin grin on his face.
1970		E. Thompson Garden of Sand (1981) 315: That Mississippi cat-eatin-shit-grin.
1971		T. Thackrey Thief 191: She still hadn’t lost that shit-eating grin.
1974		C. Loken Come Monday Morning 148: Andy smiled his shit-eatin’ smile leaned in patted Russ on the shoulder.
1979		E. Torres After Hours 184: I could see the shit-eatin’ grin on Dave’s face.
1982		W. Wharton Midnight Clear 154: You have that Buddha shit-eating smile all the time.
1983		S. King Christine 9: Then a large resplendent shit-eating grin appeared.
1990		R. Malan My Traitor’s Heart (1991) 85: I gave him my shit-eating grin, saying brother this, brother that.
1994		Rebennack & Rummel Under A Hoodoo Moon 205: He got up, a big shit-eating grin on his face.
1997		(con. 1960s) A. Frewin London Blues 277: I hand them [i.e. photographs] back to Desmond, who is still sporting this shit-eating grin.
2000		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.
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 445: Hombres todos — men with shit-eater grins.
2003		Chicago Trib. 17 Jan. 7A-4/3: His trade-mark crud-eating grin.
2004		J. Ellroy ‘Hollywood Fuck Pad’ in Destination: Morgue! (2004) 224: Big, burly, curly-haired, gap-toothed. Monstrous shit-eating grin.
2005		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?
2007		L. Redhead Cherry Pie [ebook] A lean man [...] wearing an open necked bodyshirt, a gold chain, and a shit eating grin.
2010		A. Steinberg Running the Books 78: [I] saw a group of inmates standing behind me. big shit-eating grins on their faces.
2017		D. Winslow The Force [ebook] He’s got a big shit-eating grin on his face.