Anus pus is when a man fucks either a man or a female in the asshole, and cums inside it.
After walking in a dysfunctional manner for a few days, the cum inside your anus becomes raw cum and slowly soothes its way out of your butthole in a pus like formation.
Man: Did you like being fucked in the ass by my huge raging cock?
Woman: Oh yes sir, it was marvelous. But I think I have anus pus.