under breath

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Related to under breath: fall short, slink around, so much for

under (one's) breath

Of speaking, very softly or indistinctly, especially so that one's words can only be heard or understood by oneself. "You'll get what's coming to you one of these days," Janet muttered under her breath. Quit speaking under your breath like that, Steven. Speak up so that I can hear what you're saying!
See also: 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.
See also: breath