lost in thought

adj. 沉思(神往, 呆呆地想着)

例句与用法：

My sister sat silent for some time, lost in thought.

我姐姐沉默地坐了一会儿，陷入沉思。

She heaved a sigh and became lost in thought.

她叹了口气，随即陷入了沉思。