Dictionary / Vocabulary / 口が減らない
くちがへらない
fast-talking; always having another argument ready; not able to keep one's mouth shut; never at a loss for words
Also written 口がへらない
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.