The added sexual attractiveness to another as a result of them being forbidden to you. This is anything from teachers to step-mothers, or your best mate's sister. Incest also comes in to this, but that's pretty ill-advised if you ask me.
"Dan's mum just rocks my world, she's really a 5/10 but that forbidden fruit factor bumps her up to a solid 9"
by FretWizard May 1, 2015