Possible reasons for the shift from "hunter" to "idiot":

One suggestion is that Bugs Bunny's references to Elmer Fudd as a "poor little Nimrod",[1] while most likely using the term's "hunter" sense, contributed to the development of a sense "one who is easily confounded".

An alternative explanation of this sense is that it derives from the John Steinbeck memoir Travels with Charley: In Search of America, in which Steinbeck used the term sarcastically while describing an inquest that was held after a hunter accidentally shot his partner: "The coroner questioning this nimrod..."[2]

The Oxford English Dictionary, in turn, cites a 1933 writing as the first usage of nimrod to refer to a fool, predating Bugs Bunny by at least five years and Steinbeck by nearly thirty: in Hecht and Fowler's Great Magoo, someone remarks "He's in love with her. That makes about the tenth. The same old Nimrod. Won't let her alone for a second."[3] However, this could still have been used in the sense of a hunter (i.e. someone pursuing a love interest).

The Merriam-Webster Dictionary suggests that because the legendary Nimrod was associated with the Tower of Babel, a disastrous idea, nimrod acquired the meaning of "a stupid person."[4]

Another possible source of the sense is the play "The Lion of the West" by James Paulding. First performed in 1831, it features a comedic characterization of Davy Crockett named Col. Nimrod Wildfire who attempts to woo a young French woman.