under one's breath

Idioms  

1. 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.

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Example Sentences

"Much as the Arsenal kit does indeed provoke eye bleed, this can be ameliorated by singing the theme tune to Roobarb and Custard under one's breath, as this horror of a kit is trimmed in an iffy maroon," suggests Allison Sandford.

From The Guardian

My dear Princess," she said, "your tea has been so good, and the company of your young compatriot has been so charming, that I have done nothing but chatter, chatter, chatter away about things which should only be spoken of under one's breath, and now I must hurry away.

From Project Gutenberg

Here, of course, one can only speak of Calvin under one's breath, but some day, when we are more private," and Mrs. Church looked round the room, "I will give you my view of him.

From Project Gutenberg