sincere (adj.) — 1530s, “pure, unmixed, unadulterated;” also “free from pretense or falsehood,” from French sincere (16c.), from Latin sincerus, of things, “whole, clean, pure, uninjured, unmixed,” figuratively “sound, genuine, pure, true, candid, truthful” (unadulterated by deceit)

genuine (adj.) — “natural, not acquired,” from Latin genuinus “native, natural, innate,” from root of gignere “to beget, produce”

authentic (adj.) — “authoritative, duly authorized” (a sense now obsolete), from Old French autentique “authentic; canonical” (13c., Modern French authentique) and directly from Medieval Latin authenticus, from Greek authentikos “original, genuine, principal,” from authentes “one acting on one’s own authority,” from autos “self” (see auto-) + hentes “doer, being”