Cotton panties or underwear that have been farted in so many times that the fabric near the anus becomes partially transparentto the point where the spot is distinctive.
Sunderwear is never shit stained nor is it created by direct contact with poo matter. Rather it is created by the various chemicals in the fart acting upon the underwear fabric.
A pair of particularly scandalous or slutty panties; like the tiny, black lace, completely see-through ones that you feel awkward washing with the rest of your clothes because they are just that sexual, or Heaven forbid you use a public laundry room and they should fall out of the pile onto the floor before a crowd of judgmental eyes...or your mom.
I was cold, and on a leasurely jaunt with my lady friends, and to aid my temperature driven ailment they all removed their pants, and tired their underwear into a scarf. So I was all like, "Thanks for the scunderwarf."