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1.
A boner that curves upward, resembling an upturned banana. Legend has it it's especially good at stimulating the elusive g-spot on the female anatomy, but I wouldn't know because I have a straight boner.
The banana boner is a mystery! How can a curved biological structure be so rigid?!
by Darren Key June 26, 2005
 
2.
When a male has such a large erection, (Particularly during sex), that his penis begins to curve upward to resemble the shape of a banana.
Last night was so great, that I got a banana-boner.
by SE123 March 10, 2014