if you say something under your breath, you say it very quietly so that people cannot hear the exact words.
'I don't believe you.' she muttered under her 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.
under your breath
in a very quiet voice; almost inaudibly.
under your ˈbreath
in a whisper (= a low voice), so that others cannot hear: He muttered something under his breath. OPPOSITE: out loud