Green’s Dictionary of Slang

(US campus) a toadying or hypocritical smile.

1967

D. Simmons ‘Terms Used in a Men’s Dormitory’ AS XLII:4 230: seg, n. A mythical substance reputed to be on anyone’s face if he smiles as if he were extremely pleased either with himself or with something that has just happened [...] an acronym deriving from the phrase shit-eating grin. ‘Wipe that seg off your face.’.