waste one's breath
Speak in vain (because no one agrees), as in Don't waste your breath complaining to the supervisor—it won't help. This notion was first recorded about 1400 as wasting words. The exact idiom was first recorded in 1667. Also see save one's breath.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.