A Mexican Thong is when a man is fucking a woman in the ass while eating a taco. The man then pulls out quickly so she shits everywhere, then the man rubs the shit up her crack and around her waist like a thong and then proceeds to play the mexican hat dance.
Tommy: "Dude, I'm going to break up with my girlfriend tomorrow, but first, tonight I'm going to give her a Mexican Thong!"
Aaron: "Dude! Nasty!"