The anus of a fish which, given that fish spend all their lives in water, would have to provide a very tightseal. 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 neverleave a tip when I eat out- even if the service was excellent".
Jerry: "Man, you are tighter than a fish's asshole".
Someone who is addicted to obtaining money and building wealth. A money addict and fanatic. Breadheads often work more than one full-time job, and some even participate in illicit activities to "obtain the bread".