be on to

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be on to (someone or something)

1. To have knowledge, be suspicious, or be aware of someone's actions, behavior, or intention. You'd better tread carefully, I'm on to you now. The police are on to us! Everybody run!
2. To be in contact with someone at a given point in time. I'll be on to you tomorrow about the details of our meeting. I was on to John yesterday about where he wants to go for his birthday.
3. To discover, realize, or be in the process of doing something of great importance, value, or insight. Great work on this essay, Lindsay. I really think you're on to something here. Scientists now believe they may be on to a cure for cancer.
See also: on
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

be on to

1. Be aware of or have information about, as in They can't pull that trick again; we're on to them now. [Colloquial; second half of 1800s]
2. Discover something important or profitable, as in The researchers claim they are really on to something big. [Colloquial; mid-1900s]
See also: on
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

be on to

Informal
To be aware of or have information about: You'll never deceive us again; we're on to you.
See also: on
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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