From Longman Dictionary of Contemporary Englishlet on phrasal verb informalTELL A SECRETto tell someone something, especially something you have been keeping secretlet on (that) Don’t let on that I told you.let on who/why/how etc We never did let on how we found out. I’m sure he knows more than he’s letting on. → let→ See Verb table
Examples from the Corpus
let on (that)• But she doesn't know, so don't let on.• Is this girl trained to know more than she lets on?• But she never let on about her boyfriend's criminal past.• That man down there knows more than he lets on - always has, always will.• More than is let on, because their victims are often too embarrassed to own up.• Minutes are produced from the meeting and the project is let on least cost.• Don't let on that I told you.• But he forgot to let on that it was still being built - and didn't have any beer.• And you never let on what you could manage to do if pressed. 