Loose-lipped; talkative — someone who cannot keep secrets and tends to blurt things out.
彼は口軽で、会議の内容をすぐ人に話してしまう。
He's loose-lipped and immediately tells others what happened in meetings.
口軽な人には重要な情報を預けられない。
You can't entrust important information to someone who is loose-lipped.
飲み会で酔うと口軽になってしまうことがある。
Sometimes people become loose-lipped when they drink at parties.