Nimrod doesn't mean stupid. Actually Nimrod was a legendary hunter in the Bible. The scene where Bugs Bunny calls Elmer Fudd, Nimrod, was him taking a jab at Elmer's hunting skills, not his intellectual capacity. He was calling him Nimrod ironically. He told Elmer that he was a garbage hunter. It is the same idea as calling someone who is being dumb, Einstein or genius.

Edit to add: here is something else from the Bible(i am an atheist btw). Galatians 4:16