Green’s Dictionary of Slang

(orig. US) to talk ceaselessly.

1983		Newsweek (Nexis) 17 Oct. 58: J. D. liked him for his education and his worldliness, for the way he could talk intelligently about politics or sophisticated movies while everyone else was motor-mouthing full time about the crimes they were going to do when they got out .
1996		J. Logan Tornado [film] Y’all gonna motormouth all day or are you gonna eat barbecue? [HDAS].
2002		‘Paperbacks’ Indep. 7 Dec. 🌐 Motormouthing from the comfort of his Washington home [...], he targets the usual suspects: UN ambassadors, Hillary Clinton and Democrats in general.
2021		J. Ellroy Widespread Panic 177: The witch [...] motormouthed till 3:00.