Questions on word origins and changes in meaning are right up the alley of /r/etymology, so you might try x-posting there. Meanwhile, here's what the Online Etymology Dictionary has to say on the matter:
Nimrod: "great hunter," 1712, a reference to the biblical son of Cush, referred to (Gen. x:8-9) as "a mighty hunter before the Lord." It came to mean "geek, klutz" by 1983 in teenager slang, for unknown reasons. (Amateur theories include its occasional use in "Bugs Bunny" cartoon episodes featuring rabbit-hunting Elmer Fudd as a foil; its possible ironic use, among hunters, for a clumsy member of their fraternity; or a stereotype of deer hunters by the non-hunting population in the U.S.)
According to Garner's Modern American English (Oxford University Press, 2009), astonishingly the change can be attributed to a Bug Bunny cartoon. Bugs ironically refers to Elmer Fudd as "Nimrod", comparing him to the great hunter while mocking him as a poor hunter. Garner's claims that reference the extremely popular cartoon was misunderstood as "idiot" which over time lead to the change. Garner's also suggests that the cartoon also led to the the decapitalization of the word. The author argues that this is an example of Stage 4 verbal change.
As a side note that you may find interesting, it is a relatively common first name in Israel