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1590s, "natural, not acquired," from Latin genuinus "native, natural," from root of gignere "beget" (see genus), perhaps influenced in form by contrasting adulterinus "spurious." [Alternative etymology is from Latin genu "knee," from a supposed ancient custom of a father acknowledging paternity of a newborn by placing it on his knee.] Meaning "really proceeding from its reputed source" is from 1660s. Related: Genuinely; genuineness.