In most English-speaking countries, Nimrod is used to denote a hunter or warrior, because the biblical Nimrod is described as "a mighty hunter". In American English, however, the term has acquired a derogatory meaning of "idiot"; there are various hypotheses as to why. Most examples suggest an intermediate form where Nimrod is used deliberately to mock a hunter. Whether this usage was widespread, or how it influenced the final meaning where the hunter connotation is unintended, might be beyond reach.
One suggestion is that Bugs Bunny's references to Elmer Fudd as a "poor little Nimrod", 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..."
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." 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."
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.
Another possibility is that there was an unattested dialectal or slang term for an idiot similar to Australian English ning nong, and that it became conflated with the more respectable term, perhaps as a euphemism.
nimrod (plural nimrods)