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definitions
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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]

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2

Discover something important or profitable, as in The researchers claim they are really on to something big. [Colloquial; mid-1900s]

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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.

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