A blabbermouth is someone who can't keep secrets or talks too much about things they shouldn't share. If you tell a blabbermouth that you're planning a surprise party for your friend, they'll probably spill the beans before the big day. If you confide in a blabbermouth about something embarrassing that happened to you, they might tell half the school by lunchtime.

Being called a blabbermouth isn't a compliment. It means people see you as unreliable with private information. Everyone occasionally shares something they shouldn't, but a blabbermouth does it regularly. They might not mean harm, but they struggle to keep quiet about juicy information or exciting news, even when they've promised not to.