Nimrod is described in Genesis as "the first on earth to be a mighty
man" and "a mighty hunter before the Lord." It's easy to see how people
made the leap from one mighty hunter in the Bible to calling any hunter a
nimrod. A lesser-known fact is that nimrod has seen
some use in English as a noun meaning "tyrant" (apparently, the mighty
Nimrod was not reputed to be an especially benevolent king), although
that sense is now essentially obsolete. The legendary Nimrod is also
sometimes associated with the attempt to build the Tower of Babel.
Because the tower resulted in the wrath of the Lord and proved a
disastrous idea, nimrod is sometimes used with yet another meaning: "a stupid person."