mutter (something) under (one's) breath
1. To say threatening, consternated, or disgruntled remarks 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 muttered under her breath. Steven stormed off to his room, muttering curses under his breath.
2. To say threatening, consternated, or disgruntled remarks 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 muttered under her breath. Steven stormed off to his room, muttering curses under his breath.