When you eat so much taco bell that you shit in your pants so bad that it forms a belt around your waist.

Large, beautiful breasts that are keep well consealed and initially appear to be much smaller and less impressive than they actually are until unleashed.

A taco belt is when you eat so much taco bell that you end up shitting your pants. At that point, it forms around your waste like a belt. Which is why its known as a "taco belt".