curse (someone or something) under (one's) breath

To say threatening, consternated, or disgruntled remarks (toward someone or something) in a very soft or indistinct voice, such that no one else can hear or understand them clearly. "You'll get what's coming to you one of these days," Janet cursed under her breath. Steven stormed off to his room, cursing his parents under his breath. My dad spent nearly an hour trying to fix the engine on the side of the road, cursing the broken-down truck under his breath the whole time.