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let (someone or something) in
To allow or permit someone or something to enter (some place or event). They don't usually let kids in, but this was something of a special occasion. Tom! Go get the door and let the dog in.
let (someone or something) in(to) (some place)
To allow or permit someone or something to enter into some place or event. Will you please let me into the show? My whole family is in there already! Don't let that stray cat in the house—it could be diseased.
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.