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Question

Let something slip (out)

to reveal a secret carelessly or by accident.

 

Ex: I didn't let it slip out on purpose. It was an accident.

John let the plans slip when he was talking to Bill.

Answer 1

also the expression ^ to let the cat out of the bag ^ can be useful sometimes.

eg. that boy did not know he was not my son. who let the cat out of the bag.