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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。