What's so funny it's not even funny?

When people say, "I'm so tired it's not even funny" or "my head hurts so much it's not even funny", why would it be funny in the first place? Unless someone is laughing, it's not even funny. It's odd. Say you're really funny and can make people lol with what you say then it's probably ok to say "Oh I'm so funny it's not even funny anymore".

But it sounds odd that's why you won't hear people say it. Is your mind getting a bit funny with this? Mine is. If I keep going, it'll make your head hurt so much it's not even funny...arrrrgh! Can something be so funny that it's not even funny?

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