gasp; under one's breath
Refers to a sharp intake of breath, often due to surprise, pain, or emotion. Can also describe speaking in a low, breathy voice.
She let out a small gasp under her breath in surprise.
He murmured 'help' under his breath.
Compound of 息 (breath) + の (possessive particle) + 下 (under, below). Literally 'under the breath'.