Expressions (phrases, clauses, etc.), Noun or verb acting prenominally
fast-talking; always having another argument ready; not able to keep one's mouth shut; never at a loss for words
see 口の減らない
ああ言えば、こう言う。まったく、口が減らない奴だ。Whatever I say, he has a comeback ready. You can't get a word in edgewise.
Also written 口がへらない【くちがへらない】