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 tight
er than a fish's asshole".