A.k.a. "going commando", sporting an unfinished basement implies wearing no under garments.
Go pants Todd, he's got an unfinished basement.
The not-completely-wiped asshole or the act of tossing the salad of someone with said asshole.
Phyllis nearly threw up when she licked Chris' asshole and discovered that he had an unfinished basement. She promptly bought him an extra roll of TP.