What does "be onto that" mean?
To be "onto something" is an idiom that means someone has discovered something or is aware that something is happening. This expression is often used in contexts where someone becomes aware of something that someone else is hiding and doesn't want other people to know about. Ex: The cops are onto us; they know we committed the crime. Ex: I tried to throw a surprise party for my mom but she was onto it from the start.
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