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1. To allow someone or something to enter some place; admit someone or something: We don't usually let the cat in the house. I opened a window to let in some fresh air. There was no answer at the door, so I got the key from under the mat and let myself in.
2. let in for To make oneself subject or vulnerable to something: I didn't know the trouble I was letting myself in for when I accepted that invitation.
3. let in on To tell someone something that has been kept private: I'll let you in on a secret if you promise not to tell anyone. They won't let me in on their plans.