Simon Pegg, who’s been promoting his autobiography Nerd Do Well, has apparently been spreading untruths about this most sturdy and sonically evocative put-down. The actor says it’s short for ne’er-do-well—both of which are terms, he says, that describe "someone on the fringe of society." However, this "seemingly authoritative claim lacks even a shred of historical evidence," writes Zimmer.
Other theories that have been floated, but dismissed: that it’s a reference to famous 1930s’ ventriloquist dummy Mortimer Snerd; that it’s drunk spelled backwards (since since a true knurd is far too studious to ever get inebriated, and is therefore the inverse of a drunk); and—my personal favorite—that it’s a portmanteau of nut and turd.
Zimmer's article runs through a bunch of other amusing hypotheses. He gives some wary credence to the theory that the fantastical nerd in Dr. Seuss’s 1950 book If I Ran the Zoo gave rise to the insult, though he notes that that would mean the term had a very quick adoption: The first known appearance of the word came in a 1951 Newsweek article that cited it as an example of contemporary teenspeak. But in the end, Zimmer concludes that there probably isn't a single, unambiguous source for the term.
The reasons for nerd's longevity, though, seem somewhat clearer: It's probably due to its brevity—a key characteristic of slang words that stick around, as Juliet Lapidos noted in Slate a few weeks ago—though it lacks the "back-of-the-mouth" noises that characterize other tenacious slang words, like cool and booze. In this case, however, the nasal, front-of-the-mouth sound probably helped cement nerd's place on the linguistic landscape: After all, you can’t help but sneer when you call someone a nerd.