Being an adopted child of no blood relation to one's adopted parents, the act of having sex with a relative. It has all the flavor of incest without the incest, technically since there is no close biological link between the two (or more) individuals partaking in the act.
Mike: Hey, Nick, did you hear Brian fucked his older sister?
Nick: Awww, gross, dude!? What the fuck?
Mike: I know, but technically they're not related. Brian is adopted; so, it was diet incest.
Nick: Awww, gross, dude!? What the fuck?
Mike: I know, but technically they're not related. Brian is adopted; so, it was diet incest.
by jmw87 October 1, 2010