Nimrod was an ancient Babylonian king known in part for his tyrannical rule and for his skill and might as a hunter. From this we can deduce the traditional definitions of the word nimrod: (1) a tyrannical ruler, and (2) a skilled hunter.
In late-20th-century American usage, however, the word gained a newer, much different sense: a bumbling or stupid person. The exact origins of this newer nimrod have not been definitely established. It’s been suggested that it developed from earlier, ironic use of the word to mock inept hunters. Or it could just be that nim sounds like dim while rods are not known for their intelligence.
In any case, the newer use was rare until the 1990s (though a handful of earlier examples, some from as far back as the 1930s, have been noted in the OED and on other websites). Since then, the newer nimrod has largely supplanted the older one in the U.S., especially in informal contexts, but it hasn’t gained much traction elsewhere. And even in the U.S., it is not very common and lacks some of the bite of reliable synonyms like idiot and moron.
These Nimrods grew great by the Strength of their Limbs, and their Vices; engraved their Murders upon their Shields, and hectored all the little and peaceable People into Peasantry. [The Gentleman’s Library (1744)]
He did not ride well, which surprised us, as, from the frequent allusions to horsemanship in his works, we expected to find him almost a Nimrod. [New Monthly Magazine (1832)]
They are a cheerful lot, illy concealing the glee of a Nimrod who has just snagged big game. [The Horse and Buggy Doctor, Arthur E. Hertzler (1938)]
If there’s a nimrod in the chat room and you don’t want to see their comments, you can block them by clicking on their name. [Amazon.com for Dummies, Mara Friedman (2003)]
There are no Discovery Channel scriptwriters feeding him lines, no contrived, for-the-camera flare-ups, no celebrity clientele dropping by-hell, not even a nimrod son to take out the trash! [Cycle World Magazine (2004)]
[I]t is ridiculous that a handful of local nimrods can pass laws that violate their civil rights. [comment on Washington Post (2008)]
If so, you’re not a contrarian — a bold thinker who does things differently to get better results. You’re a nimrod. [Reno Gazette-Journal (2013)]