let slip

verb2

1. •
   To divulge a secret, as by accident or mistake.

   • It was let slip that they were planning to take over the business.
2. •
   (slang, no longer used) To let fly; to attack or challenge someone.

   • I would take it kind of your worship to let me be present at his examination, that if he clear himself of one murder I may let slip at him with the other. — 1862

verb1

1. •
   To accidentally reveal a secret.

   • We were talking about Natalie when I let slip that she was pregnant.