Under your breath - Idioms by The Free Dictionary
under your 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