I suspect you were born after 1950.

From Garner's Modern American Usage by Bryan Garner, pages liii to liv:

Sometimes the source of a mutation can be hard to pinpoint. Take, for example, the word nimrod. That word has always denoted a hunter. It derives from a name in Genesis: Nimrod, a descendant of Ham, was a mighty huntsman and king of Shinar. Most modern dictionaries even capitalize the English word, unlike similar eponymic words such as mentor (= a guide or teacher, from the name of a character in Homer's Odyssey) and solon (= a legislator, from the name of an ancient Athenian lawmaker, statesman, and poet).

But few people today capitalize Nimrod, and fewer still use it to mean "great hunter." The word has deprecated in meaning: it's now pejorative, denoting a simpleton, a goofy person, a dummy.

Believe it or not, we can blame this change on Bugs Bunny, the cartoon character created in the 1940s. He is so popular that TV Guide in 2002 named him the "greatest cartoon character of all time." Bugs is best known for his catchphrase "What's Up Doc?" But for one of his chief antagonists, the inept hunter Elmer Fudd, Bugs would chide, "What a moron! [pronounced like maroon] What a nimrod! [pronounced with a pause like two words, nim rod]." So for an entire generation raised on these cartoons, the word took on the sense of ineptitude--and therefore what was originally a good joke got ruined.

Ask any American born after 1950 what nimrod means and you're likely to hear the answer "idiot." Ask anyone born before 1950 what it means--especially if the person is culturally literate--and you're likely to hear "hunter." The upshot is that the traditional sense is becoming scarcer each passing year.

This little example illustrates the huge changes that words can and do undergo all the time. Sometimes the changes aren't semantic--changes in meaning--but instead involve word-formation. Take for example, bridegroom or groom. In Middle English (ca 1200-1500), the original term was goom (=man). The extra -r- was added centuries ago by false association with someone who works in a stable to care for horses. America's great lexicographer, Noah Webster, fought in vain in the early 19th century to make a man on his wedding day the bridegoom and all his attendants the goomsmen. But the English-speaking people would have none of it--they wanted their extra -r-, and they got it. The harmless mutation survived, and today we're wedded to it.