Under your breath
Meaning: If you say something under your breath, you say it very quietly so that other people cannot hear it. So softly that almost no one can hear it. (*Typically: curse ~; curse some-one or something ~; mutter ~; mutter something ~; say something ~.)
Example: 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.
Example: 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.