have something on
have something on {v. phr.}, {informal}
To have information or proof that someone did something wrong.
Mr. Jones didn't want to run for office because he knew the opponents had something on him.
Mr. Smith keeps paying blackmail to a man who has something on him.
Although Miss Brown is not a good worker, her boss does not fire her because she has something on him.
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Source: A Dictionary of American Idioms