waste of breath

A waste of space or life.

Stacey's so stupid, she's such a waste of breath!

Waste One’s Breath

An idiom, Used when someone says something that will probably be ignored.
See also: Waste your breath.

Person 1: Omg! That girl’s hair looks ridiculous! Just look at it..
Person 2: don’t waste your breath talking about her hair, she probably likes it!
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Example 2
Person 1: I think your clothes have stains on it..
Person 2: Don’t waste one’s breath! I’ll change my clothes.