Definition of "let in"

let in

verb
third-person singular simple present lets in, present participle letting in, simple past and past participle let in
1. (transitive) To let someone or something come in; to admit someone or something in.
  1. To divulge one's inner thoughts to (someone), making oneself emotionally vulnerable to them; to open up to (someone).
    Quotations
    So tell me when you're gonna let me in / I'm getting tired and I need somewhere to begin
    2004, “Somewhere Only We Know”, in Hopes and Fears, performed by Keane
2. (transitive, Oxford University slang) To associate with.
  Quotations
  He has also been good enough to recommend to me many tradesmen who are ready to supply these articles in any quantities; each of whom has been here already a dozen times, cap in hand, and vowing that it is quite immaterial when I pay—which is very kind of them; but, with the highest respect for friend Perkins (my scout) and his obliging friends, I shall make some enquiries before "letting in" with any of them.
  1859, Thomas Hughes, chapter 1, in Tom Brown at Oxford
3. (UK, slang, obsolete) To cheat or victimize.