What does “be onto someone” mean?
To be "onto someone" in this case is an expression that means that you have knowledge or are suspicious of what someone is doing. Usually, it refers to when someone is doing something sneaky, or something they don't want others to know about. Ex: I tried to make up an excuse for not coming to class today, but my teacher is onto me. Ex: He thought he could get away with his crime, but the police is onto him. Ex: We wanted to throw her a surprise party, but she's onto us.
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