From Longman Dictionary of Contemporary Englishunder your breathunder your breathQUIETin a quiet voice so that no one can hear you ‘Son of a bitch, ’ he muttered under his breath. → breath
Examples from the Corpus
under your breath• And, as he played, he seemed to talk to himself under his breath.• Sitting back, humming under his breath, he scanned the waters for anything that might present itself.• "Son of a bitch, " Bill muttered under his breath.• Greatly relieved, he muttered under his breath and crossed himself several times.• Major Roland Tuck swore peaceably under his breath.• Quietly, under my breath, I began humming Handel.• Peter muttered something resentful under his breath, but did as she asked.• He swore under his breath and then quickly thrust the sack back into the water.• He hissed those words under his breath, your friend, his fingers digging mindlessly into the clear plastic packets of prophylactics. 