So I assume that "hail" can be used to pay respect to someone. But can I only use it for a monarch?
Yes. In this context, the word “hail” is used to respectfully greet the Monkey King. The term was used in the past when addressing monarchs and leaders. It would be exclaimed by an individual and repeated by the crowd. It is not common to use the word “hail” now when greeting ordinary people or monarchs. Ex: All hail the Queen of England. Ex: Hail, Caesar! Emperor of Rome.
Get more Native English Expressions >