curse (someone) under (one's) breath
To murmur something in such a soft, quiet voice that others cannot hear it distinctly. (Said especially of rude, unpleasant, complaining, or impertinent remarks.) I could hear Bill cursing the boss under his breath as he walked back to his desk after his annual review. If you curse under your breath around me again, I'll tan your hide so quick it will smart for a week!