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A kacknerd is a person who acts like they know everything but is actually full of hot air and dumb as a sack of rocks.
My math teacher is a kacknerd. He thinks he's Einstein but can't even add 2 + 2 without a calculator.
That kid in class called me a kacknerd because I got a better grade than him on the test.
My cousin is a kacknerd. He said he could fix my car, but now it's broken and smells like burnt toast.