Erection Pants are pants that don't fit in the crotch area and are too loose/baggy. This causes the fabric at the crotch to bunch up when the person sits down, making him/her look like he/she has an erection.
Person 1: "Wow, Tina lost a lot of weight!"
Person 2: "Yeah, you can tell just by looking at her erection pants."
Person 1: "These jeans just don't fit! They're so baggy!"
Person: "You have a pair of erection pants, my friend."