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1.
an imaginary fish from JD Salinger's short story called "A Perfect Day For Bananafish" which is found in the book 9 stories by Salinger. According to the main character of the story banana-fish are fish the swim into coral reef and holes in rocks to eat the bananas growing there but then eat too many and cannot swim back out and then die because of this.
The story about bananafish is rather sad.
by bogus January 19, 2005
 
2.
1. Something that seems like a good idea at the time, but turns out to be a terrible mistake.

2. Something that others try to warn you not to do, but you don't listen, and it turns out to be a terrible mistake.
"That idea sounds a lot like banana fish to me." or, "Don't do that! It'll turn out to be banana fish!"
by The Garrats October 01, 2006