I read somewhere a theory that the change of meaning in "nimrod" from hunter to idiot was caused by Bugs Bunny/Elmer Fudd cartoons, as Bugs jokingly calls Elmer Nimrod a few times in the hunting cartoons, and people who didn't know what it meant would take it as a random insult.
smartalix suggests: 'It could also have devolved into insult the way the term "Einstein" has, an example of hyperbolic sarcasm. Today calling someone "Einstein" usually means that they are a moron.'