let in on

let in on idiom
Allow someone to know about or participate in something, as in I'm going to let you in on a little secret. This idiom was first recorded in 1904.

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.

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