waste breath

To talk of, discuss, or say things that are likely to be ignored or are in vain. Don't bother trying to change my mind about this, you're wasting your breath! It looks like I wasted my breath trying to pitch my idea to the board of directors.

Fig. to waste one's time talking; to talk in vain. Don't waste your breath talking to her. She won't listen. You can't persuade me. You're just wasting your breath.

To gain or accomplish nothing by speaking.

To talk in vain, because no one will listen. The idea that breath is something that can be saved or wasted dates from the sixteenth century (see also save your breath). Tennyson used the term in “In Memoriam” (1850): “I trust I have not wasted breath.”