under one's breath

under (one's) breath

Spoken in such a soft, quiet voice that others cannot hear it distinctly. "You'll get what's coming to you one of these days," Janet muttered under her breath.
See also: breath
Farlex Dictionary of Idioms.

under one's breath

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.
See also: breath
The American Heritage® Dictionary of Idioms by Christine Ammer.

under one's breath

In a whisper or an undertone. Presumably this term is a hyperbole for something softer than breathing. It dates from the early nineteenth century. Edward Bulwer-Lytton used it in Eugene Aram (1832): “Hush, said Ellinor under her breath.”
See also: breath
The Dictionary of Clichés by Christine Ammer
See also:
  • under (one's) breath
  • under breath
  • under your breath
  • curse (someone) under (one's) breath
  • mutter
  • mutter (something) under (one's) breath
  • with every (other) breath
  • with every breath
  • that will do
  • below (one's) breath