A motormouth is a person who talks and talks and talks — fast, loud, and without stopping long enough for anyone else to get in. The picture inside the word is that the mouth has an engine in it: switched on, running by itself, and hard to turn off. It is informal and slangy, mostly American, and you use it about people, usually with a sigh or a laugh: 'don't sit next to Kevin, he's a total motormouth.' It can be affectionate between friends, but said to someone's face it stings, so it is safer used behind their back or about yourself. Notice what it does and does not say. It is about the amount and speed of the talking, not the content — a motormouth may be very clever and very entertaining. It is not the same as a liar, a gossip, or a bore. Sports commentators and radio hosts who speak very fast get called motormouths half in admiration. There is also a verb, 'to motormouth', meaning to talk on and on like that, but it is much rarer; most people only ever use the noun.

話很多、講不停的人；台灣會說「嘴巴像機關槍」「話匣子一開就停不下來」。是口語、帶點調侃的說法，當面講會傷人。