Not dumb. Derivations of expressions change (or maybe get hijacked!). As I’ve understood it from many decades of use: nut case is like nutty/crazy/etc and case is like a medical case, as in a case of chicken pox. Put the two for the common, traditional use. Somebody has gone a bit anatomical with this new, contrived derivation. And in any case, I do not know what behaviour one should attribute to somebody behaving like a scrotum - answers on a postcard, please…