Kevin: Come on, tell me! I told you my secret.
Penny: No, I can’t tell you. You’re a big blabbermouth. You already told everyone about the time I threw up at the dentist’s office.

To blab is to give away the secrets of your friends or allies. Some people are just no good at keeping secrets. It is not a nice thing to say, but we call them “blabbermouths”. It is best not to tell them anything that you don’t want everyone else to know. If they blab maliciously, they might also be called “gossips”.