ratchet mouth, n. — Green’s Dictionary of Slang

ratchet mouth n.

also rachet jaw

[SE ratchet, a notched wheel used in a machine + mouth/jaw]

a chatterer.

1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 212: ratchet jaw, n. – someone who talks too much.
1977		1977 World Book Year Book 267: Overcrowding, which can turn ‘good buddies’ into nasty rivals. CBers are supposed to limits calls to five minutes and those who do not are called ‘ratchet jaws’.
1986		R.L. Chapman NDAS 354/2: ratchet-mouth or ratchet-jaw n. A person who is constantly talking.