Etymology
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blab (v.)
mid-15c., blabben, "to talk idly and foolishly, talk too much," apparently from Middle English noun blabbe "one who does not control his tongue" (late 13c.), which is probably echoic. Related: Blabbed; blabbing. From c. 1600 as "to talk indiscreetly."

The exact relationship between the noun and verb blab and blabber is difficult to determine. The noun was "[e]xceedingly common in 16th and 17th c.; unusual in literature since c 1750" [OED]. Middle English also had lab (v.) "talk foolishly, let out a secret" (late 14c.), said to be from continental Low Germanic; hence also labster (Middle English labestere, late 13c.) "female gossip, a scold."
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mouth (v.)

early 14c., "to speak," from mouth (n.). Related: Mouthed; mouthing. Old English had muðettan "to blab." In 17c.-18c. especially "to speak pompously or affectedly." Meaning "form the shape of words with the mouth without uttering them" is by 1953.

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blabbermouth (n.)
also blabber-mouth, 1931, from blabber + mouth (n.).
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blabber (v.)
mid-14c., "to speak as an infant speaks," frequentative of blabben, of echoic origin (compare Old Norse blabbra, Danish blabbre "babble," German plappern "to babble"). Meaning "to talk excessively" is from late 14c. Related: Blabbered; blabbering.
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