To talk at great length without saying anything meaningful; to ramble or chatter foolishly.
From Old Norse 'bladhra' meaning 'to bleat' or 'make noise.' The word entered Middle English and took on the sense of meaningless human speech, likely because bleating sounds seemed similarly pointless to listeners.
The journey from 'animal noises' to 'useless human talk' shows how language reveals what people really thought about certain kinds of speech! Comparing blathering to bleating was their way of saying 'that means nothing.'
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