There's definitely at least one Bugs Bunny/Elmer Fudd cartoon in which Bugs refers to another character as "Nimrod." And, yes, Nimrod was a biblical hunter. read more
Thus it may have come about from the cartoon character Bugs Bunny first used as an expression of pity to mock, which may have later been used as an insult from teenage slang for unknown reasons. Hope that answers your question. read more
Because of Bugs’s popularity, the bunny’s use of nimrod became, over time, the predominant version of the word—so much so that, today, few people even know it is actually a Biblical reference. Bugs Bunny’s reshaping of the meaning “nimrod” is a prime example of how life imitates art. read more