Why is the middle finger offensive?

No amount of description or logic can change my mind on how odd it is to think on how it doesn't make sense on why its offensive or insulting, think of it, it's just someone pulling up a finger at you and for some reason if it's the index finger, people expect you to say something smart but when it's the middle one, holy shit you are a damn asshole, you pulled out the longest finger on your hand with the rest curled into your palm and watch out, the finger is alone.
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Why is the middle finger offensive?
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