To blab means to carelessly reveal information that was supposed to stay secret. When someone blabs, they let slip details they should have kept quiet about, like accidentally mentioning a surprise party to the person it's meant for, or telling your whole lunch table about something a friend shared with you in confidence.

The word suggests talking without thinking about the consequences. If your sister blabs about the birthday gift you bought for your mom, she's ruined the surprise by speaking too freely. Someone might blab a secret deliberately to cause drama, or simply blab it by talking too much without realizing what they're doing.

Blabbermouth is what people call someone who can't keep secrets or talks excessively about things they shouldn't. When your teacher says, “Don't blab about the test questions to the afternoon class,” she means don't spoil things by revealing what you know.

The word has a loose, sloppy sound that matches its meaning. Blabbing isn't the same as deliberately betraying someone's trust (that might be called snitching or tattling). It's more about being careless with your words, letting information spill out when you should have kept your mouth shut.