under one's breath

under one's breath idiom
Softly, in an undertone or whisper, as in “I can't stand one more minute of that music,” she muttered under her breath. This idiom, first recorded in 1832, is probably a hyperbole, alluding to a sound that is softer than breathing.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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