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
Fig. [spoken] so softly that almost no one can hear it. (*Typically: curse ~; curse some-one or something ~; mutter ~; mutter something ~; say something ~.) John was muttering something under his breath, and I don't think it was very pleasant. I'm glad he said it under his breath. If he had said it out loud, it would have caused an argument.