He spent so much time on the project that he's now hip-deep in it.

hip-deep 是一个复合形容词，由 hip（臀部）和 deep（深的）组成，字面意思是"深及臀部的"。 这个表达用来比喻某人或某物"深深陷入某种状态或情况中"，通常指投入程度很深或难以脱身的状态。