Fish is usually used to describe a loser. One may be called a fish if he stinks, has no friends or is just all around a terrible person. A fish could also be a term used for a stinky slimy animal. You should not make contact with a fish in order to maintain a stink free life.
girl: "ew it stinks.."
boy: "oh it must be a fish"
or
girl: "you didnt talk to that fish did you?"
boy: " oh hell no."