One tends to retain bugs up one's but when one has a rubber asshole. There must be some stretch, but for the most part, rubber assholes would be as water-tight as a frog's.

I had a boss who was very sick and discribed himself as feeling "like hammered dogshit." (Looked like it, too.) A friend occasionally says he feels like a sack of doorknobs. I'm not sure exactly what it means.