expression
it's a waste of words; there's no point in saying
Used to dismiss the idea of speaking because it would be futile or have no effect. Often said when the listener won't listen or the outcome is already decided.
彼に注意しても言うだけ無駄だよ。
It's a waste of breath to warn him.
今さら文句を言っても言うだけ無駄だ。
There's no point in complaining now.
言っても無駄 is a more flexible phrase meaning 'even if you say it, it's useless,' while 言うだけ無駄 is a fixed expression emphasizing the wastefulness of speaking.
A set phrase composed of 言う (to say), だけ (only/just), and 無駄 (waste/futility). The literal meaning is 'just saying is a waste.'