To reveal secrets or private information carelessly or indiscreetly. To talk too much about something that should remain confidential.
From Middle English blabben, possibly imitative of meaningless chatter or babbling sounds. The word evolved from describing general excessive talking to specifically meaning the careless revelation of secrets by the 14th century.
The word 'blab' is part of a fascinating family of onomatopoetic words that mimic the sounds of speech - like 'babble,' 'blabber,' and 'blather.' Interestingly, the related word 'blabbermouth' didn't appear until the 1930s, showing how the concept of gossipy revelation became increasingly important in modern society.
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