The banana plug seems to be a German invention (German Wikipedia), although English Wikipedia gives an alternative American origin.
But neither article discussed the origin of the word. It may be that I could find the origin if my German were better, but the only etymology I can find in English is in bright hub where it says:
Why Are They Called Banana Plugs?
First of all, what’s with that funky name? Banana plugs are small cylindrical plugs that have a very distinctive bulged prong. This bulge houses a spring that, when inserted into a socket, presses outwards to ensure a solid and secure connection. The plug takes its name from the shape of this prong, which reminds many people of a straightened out banana.
But I think this is a wild guess, as they offer no sources and it does not make sense. When we say something is banana shaped we are referring to its curvature - a straightened banana simply means "straight" - like all pins on plugs!
So can someone with better German than me please help?