Wordorigins.org

The use of nimrod to mean an idiot or inept person is often said to come from young viewers misinterpreting a 1940s Bugs Bunny cartoon, but that doesn’t appear to be the case. A Warner Brothers cartoon does play a role in the word’s history, but it’s not Bugs Bunny, but rather Daffy Duck, who utters the word. Nor did Daffy invent the insult, rather it comes from a slow change in the word’s meaning over the years.