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.
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.
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.”