Waste your breath
Definition 1 of 1
Pro Tip 1/3Figurative Meaning
The phrase 'waste your breath' means using words with no result; it's not about actual breathing. 
I tried to change her mind, but I was just wasting my breath.
Pro Tip 2/3Stubborn Audience
Use 'waste your breath' when someone won't listen or change their mind, despite your efforts. 
He'll never agree to that plan, so you're wasting your breath.
Pro Tip 3/3Futile Situations
The phrase suits situations where effort has no chance of success. 
Arguing about it now is wasting your breath; the decision has been made.