under your breath

this idiom, first recorded in 1832, is probably a hyperbole, alluding to a sound that is softer than breathing.

这个成语最早出现在1832年，可能是一种夸张的说法，暗指一种比呼吸还柔和的声音。

quietly so that other people cannot hear exactly what you are saying

低声地，轻声地

He muttered something under his breath.

他低声嘀咕着什么。