下山用英语可以说成 "to come down the mountain" 或 "to descend the mountain." 来源可能是源于人们在爬山时，从山上走到山下的过程。

例句：

We need to come down the mountain before it gets dark. 我们需要在天黑之前下山。

The hikers took a break to rest before descending the mountain. 登山者在下山之前休息了一会儿。