The anus of a fish which, given that fish spend all their lives in water, would have to provide a very tight seal. Metaphorically, the term is usually used in conjunction with the words "tighter than a" to indicate that someone could hardly be more frugal with his spending.
Tom: "I never leave a tip when I eat out- even if the service was excellent".
Jerry: "Man, you are tighter than a fish's asshole".
A person who, even after a prolonged period of observation, does not present one with a clear indication of whether they are male or female.
Guy 1: "Hey, see that person over there? It's too ugly to be a woman but it's wearing a floral shirt and platform shoes. Is it a guy?"
Guy 2: "And I can't tell if it's got breasts or not. The odd hairstyle doesn't help. I can't stand it when you can't tell if someone is a guy or a girl. We'll just have to classify that as a 'nonsexual'. Poor ugly bastard."