He went behind his back and applied for the promotion himself.

behind 的意思是"在...后面"，表示位置或顺序上的靠后。 his 是代词 he 的所有格形式，意思是"他的"。 back 的本义是"背部"，引申为"背后"或"背面"。 behind his back 是一个固定短语，字面意思是"在他背后"，引申为"背着他"或"在他不知情的情况下"，通常指背着某人做某事。