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late 14c., "pleasing to the imagination," present participle adjective from flatter. Meaning "gratifying to self-esteem" is from 1757. Related: Flatteringly.
early 13c., from Old French flater "to flatter" (13c.), originally "stroke with the hand, caress," from Frankish *flat "palm, flat of the hand" (see flat (adj.)). "[O]ne of many imitative verbs beginning with fl- and denoting unsteady or light, repeated movement" [Liberman]. Related: Flattered; flattering.