Inclined to talk too much; prone to blabbing or revealing secrets.
From 'blab' (to talk excessively) plus '-y' (adjective-forming suffix meaning 'characterized by'). A colloquial formation, likely from the 1800s onward.
The suffix '-y' is incredibly productive in English—you can add it to almost any noun or verb to make an adjective ('sleepy' from 'sleep,' 'rainy' from 'rain,' 'blabby' from 'blab'). It's how English speakers continuously create new words.
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