let in on
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let (one) in on (something)
1. To give one information that is private or not widely known. All right, I'll let you in on what’s really going on here—we're planning a surprise party for Carla. The boss let me in on his plans for expanding the business.
2. To include one in something, especially that to which access is secret or privileged. I've been trying to get Martha to let me in on the deal she's striking with the Ferrelli brothers. Tom's been letting us in on the special offer he gets through work.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
let in on
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.
let (someone) in on
1. To reveal (a secret) to someone: They finally let me in on their plans.
2. To allow someone to participate in (something).
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.