Under your breath.
speaking softly or muttering
The idiom 'under your breath' refers to speaking so softly or quietly that the words are barely audible, and usually indicates that the speaker is either hesitant to speak openly, or is expressing dissatisfaction or annoyance without wanting to make those feelings overtly public. When someone mutters 'under their breath,' it's usually done in a context where they wish to keep their words, often critical or personal in nature, not wholly private, but still out of general earshot.
Example Sentences
She muttered something under her breath as she walked away.
She said something quietly as she left.
I could hear him grumble under his breath about the new policy.
I heard him complaining softly about the new policy.
He cursed under his breath when he realized he'd missed the bus.
He quietly cursed when he realized he missed the bus.
