A Fat Trap is when you're on a social networking website, and see a good looking profile picture. Upon going to their profile picture, you see other photos of the person, and realize they are incredibly fat, and are actually ugly.
Person 1: Dude, check out this hot chick's profile picture!
Person 2: That is definitely a fat trap dude, look at her other pictures
Person 1: *checks pictures* oh god...thanks man, you saved me.