Slabber
A noisy, slack-mouthed talker - someone whose chatter is loud, mostly empty, and hard to switch off. Also a verb: to drool, to dribble, or to talk nonsense at length. The bodily and verbal senses share the same logic.
Etymology
From Middle Dutch 'slabberen', to lap or slaver, with the related Dutch dialect 'slabber doek' meaning a bib. Recorded in Older Scots from 1604 and in Scottish National Dictionary entries from 1712 onward. The word fell out of standard English by the eighteenth century but stayed alive in Scots and Ulster Scots, where the body-and-mouth metaphor proved durable.
In a sentence
He slabber'd me all over from Cheek to Cheek, with his great tongue. - DSL, 1712
Look at him there, the muckle slabber. - DSL, 1901
Historical notes
Slabber covers a remarkable range in DSL: drooling, eating noisily, weeping, talking nonsense, kissing wetly, a slovenly person. The Ulster Scots inheritance keeps the loud-talker noun and the babble-verb most active. The word's bluntness suits its function: 'a slabber' is a sentence in itself, an entire small judgement of someone's chat.