From Longman Dictionary of Contemporary Englishlet somebody in on something phrasal verbTELL A SECRETto tell something that is secret or only known by a few people TV chef Raymond Blanc lets us in on the secrets of his kitchen. Would someone mind letting me in on the joke? → let→ See Verb table
Examples from the Corpus
let in on • I'm going to let you in on a little secret.• Comrades, he would begin, I must let you in on a secret.• The police exhibition and campaign lets the motorist in on all the tricks including the latest speeding cameras.• One of his most effective techniques is to let us in on his writing process.• Lloyd Grossman shares the fun he's had in making the series and lets us in on some behind-the-scenes secrets.• His grandmother was letting him in on some-thing big.• But part of the pleasure of birdwatching is letting others in on the show.• Hall displays, brightly colored and often prepared by students, let everyone in on the topics at hand. 