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.

• You can say that again!
• You can talk!
• You can't judge a book by its cover.
• You can't keep a good man down.
• You can't keep a good woman down.
• You can't make an omelette without breaking eggs.
• You can't make a silk purse out of a sow's ear.
• You can't make bricks without straw.
• you can't miss it
• You can't say fairer than that.
• You can't teach an old dog new tricks.
• You can't win 'em all.
• you could cut the atmosphere with a knife
• You could have heard a pin drop.
• You could have knocked me down with a feather!
• You could have knocked me over with a feather!
• you could hear a pin drop
• you couldn't have asked for anything more
• you couldn't have asked for more
• You could've fooled me.
• You'd better believe it!
• you don't need this
• you have got to be kidding
• you haven't lived
• you haven't lived until