noun
idle talk; chatter; useless talk
Refers to talking without purpose or content, often with a negative nuance of wasting time. Commonly used in phrases like 無駄口を叩く (to chatter idly).
会議中に無駄口を叩いてはいけない。
Don't chatter idly during the meeting.
彼はいつも無駄口ばかりで、仕事が進まない。
He's always just talking nonsense, so work doesn't progress.
Compound of 無駄 (むだ, 'useless') and 口 (くち, 'mouth'), with rendaku voicing of くち to ぐち.