bite off more than one can chew
bite off more than one can chew {v. phr.}, {informal}
To try to do more than you can; be too confident of your ability.
He bit off more than he could chew when he agreed to edit the paper alone.
He started to repair his car himself, but realized that he had bitten off more than he could chew.
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Source: A Dictionary of American Idioms
