He cursed under his breath when he realized his mistake.

under 是介词，表示"在...之下"，这里指声音在某个水平之下。 his 是代词 he 的所有格形式，表示"他的"。 breath 的意思是"呼吸、气息"，指从肺部呼出的空气。 under his breath 是一个固定短语，字面意思是"在他的呼吸之下"，引申为"小声地、低声地"，指说话声音很小几乎听不见。