expression, godan verb (-ku)
chatter pointlessly; waste one's breath
Used for idle talk that serves no purpose, often with a negative connotation of being unproductive or annoying.
会議中に無駄口を叩いてはいけない。
Don't chatter pointlessly during the meeting.
彼はいつも無駄口を叩いてばかりいる
He's always just wasting his breath.