Judging by his expression, he was not pleased.

judging 是动词 judge 的现在分词形式，judge 的意思是"判断、评判"。 by 在这里是介词，表示"通过...方式/依据"。 judging by 是一个固定搭配，意思是"根据...来判断"，常用于通过观察某些迹象或表现来得出结论。