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Phrase(s): *under one’s breath
Fig. [spoken] so softly that almost no one can hear it. (*Typically: curse ~; curse someone 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.
McGrawhill's American Idioms And Phrasal Verbs