you’re nuttier than a fruitcake!!
We had a German foreign exchange student my senior year in high school....way back in the dark ages before the invention of inventing.
He considered being called “nuttier than a fruitcake” the absolute worst thing you could say about anyone. It was a major epithet in his mind. Became royally incensed over it once.
I never did figure out how it must have translated into German. Nussig wie ein fructkuchen???