It means what it means and exactly the opposite at the same time—more adventures in dictionaries
In subsequent years I started hearing the word being used pejoratively on the playground (this would be late sixties and through the seventies), and clearly the word meant either “idiot” or “dork” or “jerk.” By middle school the insult began to be a bit more sexual, but still definitely an insult, sort of a combination of “c*cksucker” and “wanker.”
Many lexicographers express skepticism that Bugs Bunny is to blame for the shift in the meaning of nimrod, but then fail to offer a compelling alternative explanation. Several of them trot out a line of dialogue from the obscure 1933 play, The Great Magoo. The word nimrod is clearly used as an insult, but it is specifically a reference to a man who had fallen in love with the showgirl, and that he’s another in a long line of nimrods pursuing her. The problem is that while it is being used as an insult, clearly the insult is still a reference to Nimrod the Mighty Hunter—in this case someone who sees this woman as a prize to be captured.
And since all of the lexicographers agree that the “synonym of idiot” meaning became common in the early 60s, it’s a little difficult to believe a play that flopped 30 years earlier was the source.
Another example that is trotted out is a series of humorous stories published in a British periodical in the late 19th Century which ran under the pseudnym of Nimrod. Each story is told in the first person and recounts another humorous misadventure while attempting to participate in a fox hunt. But that’s even harder to believe that the 1933 play, first, because of the longer period of time but also because all the dictionaries agree that the “synonym of idiot” meaning is chiefly a U.S. usage.
I’ve seen at least one person simply express skepticism that a single line of dialogue from a single short film could have the effect. I have several responses to that. First, it is three different Bugs Bunny cartoons in which the insult occurs (amusingly enough, only one of them is it used to describe Elmer Fudd, the other two times are both used against Yosemite Sam). The other thing is that from the late fifties through the seventies, Bugs Bunny was everywhere.
In 1956 Warner Brothers licensed the rights to all of their Looney Tunes cartons made up until mid-July 1948 to Associated Artists Productions. A.A.P. began syndicating them to local stations, and by 1958 were able to claim that the highest rated local shows in every metropolitan market were those that included at least some cartoons. No one had cable, and people could only get three to five local stations over the air, so your choices for entertainment were limited. And the syndication deals weren’t exclusive, so I remember that at one point in my elementary school years, where there was one show on one channel that ran every weekday morning around the time we were getting ready for school that included several Looney Tunes cartoons, plus a half hour show that ran every weekday at 4:30 on another channel that was all Looney Tunes cartoons, and another half hour of Looney Tunes that ran on a third channel every weekday at 5:30.
In addition, in 1960 Warner Brothers started producing and selling to various networks a program that combined cartoons made from mid-July 1948 on. First as a primetime weekly Bugs Bunny Show, then it moved to Saturday mornings. As I said, for a while, Bugs Bunny was everywhere.
According to at least one dictionary specializing in slang, the “synonym of idiot” meaning of nimrod was used prevalently by U.S. teens and pre-teens in the 70s and 80s. All of us kids watching Bugs Bunny cartoons in the 60s and 70s could account for the new meaning of the word arising in our age group quite nicely at that time. Whereas the obscure 1933 play and the humorous 19th Century British magazine origins just don’t make any sense as an origin for American schoolyard slang in the 70s, do they?
Finally, another reason to believe the fault lies with misunderstanding a sarcastic usage of the word is because it has happened in English many times. For example, terrific used to mean terrifying (terrific is to terror as horrific is to horror, as a friend so eloquently put it). How did terrific come to mean the opposite? Simple, the sarcastic or ironic use became far more common than the original meaning. Sometimes language just takes a left turn at Albuquerque, eh, Doc?