> "it means go bananas"

Surely it means "Balls."

"Go bananas" means "go mad", I think, although I can't derive an etymology.

"Nuts" seems to mean "mad", as in "nutty as a fruitcake", which for me brings to mind something like a Dundee cake (heavy mixture of dried fruit and nuts). I still can't derive an etymological connection between madness and (fruit|nuts).