under one's breath别理解成“喘不过气来”，其实真实意思是“轻声地，低声地，压低嗓子，低声说话”。

【例句】
I'm glad he said it under his breath. If he had said it out loud, it would have caused an argument.
我很庆幸他是低声说的。如果他大声说出来，就会引起争论。

He muttered something under his breath.
他低声嘀咕着什么。