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let in

part of speech: phrase
definition 1: to allow (someone or something) entry.
These big windows let in a lot of light.These cracks are letting in cold air.Don't let the dog in. He's all muddy.If that's the repair guy, let him in.When I first applied for membership in the club, they wouldn't let me in.
definition 2: to reveal to (someone) one's inner feelings or secret information that one has (often fol. by "on").
I'm trying to help you, but you won't let me in.They have a strategy for winning but they won't let me in on it.Come here. I'll let you in on an old family secret.
definition 3: to allow (someone) to participate (often fol by "on").
We've decided to let you in on the deal.