waste (one's) breath
To gain or accomplish nothing by speaking

[TahlilGaran] Dictionary of Contemporary English ▲

waste breath
waste (your) breath
to say something that is likely to be ignored.
You'd be wasting your breath reporting it to the police because they don't look for stolen bikes, even expensive ones.

Usage notes: often used in the form don't waste your breath:
Don't waste your breath — I've already asked her to help and she said no.

[TahlilGaran] English Idioms Dictionary ▲

waste one's breath
speak pointlessly without the desired results; waste one's time talking, talk in vain
He is very stubborn and you are wasting your breath to argue with him.

I think that I will be wasting my breath to try and ask my parents to use the car this weekend.

[TahlilGaran] English Idioms Dictionary ▲

waste one's breath
v. phr. To speak or to argue with no result; do nothing by talking.
The teacher saw that she was wasting her breath; the children refused to believe her.
I know what I want. You're wasting your breath.

[TahlilGaran] English Idioms Dictionary ▲