Nimrod was the great grandson of Noah, and he is mentioned in the book of Genesis as "a mighty hunter before the lord." So great hunters were sometimes called "Nimrod" as a compliment.
Bugs Bunny taunted Elmer Fudd, a stupid an incompetent hunter that failed to catch Bugs Bunny, by calling him a Nimrod. But the children who watched Loony Tunes didn't get the joke and so they assumed it was a synonym for "stupid and incompetent."
Imagine that in the future people think that that "Einstein" means "imbecile" because "you're a regular Einstein" was an ironic taunt. It's the same idea.
Bugs Bunny taunted Elmer Fudd, a stupid an incompetent hunter that failed to catch Bugs Bunny, by calling him a Nimrod. But the children who watched Loony Tunes didn't get the joke and so they assumed it was a synonym for "stupid and incompetent."
Imagine that in the future people think that that "Einstein" means "imbecile" because "you're a regular Einstein" was an ironic taunt. It's the same idea.