There is a phrase "The water just runs off his/her back." In this context, the image is of someone pouring water onto your back while you are fully clothed. Typically, your clothes would absorb a large amount of the water and not much would "flow" to the floor. It would be unusual for all of the water to simply flow to the ground and have none absorbed. When using this phrase, "water" typically means "problems" or "difficult events or situations." So the phrase really means "Troubling events or situations seem to not bother him." Where "water" is a "metaphor" for troubling events. "The problems seem to "flow" directly away from the person without emotion being "absorbed"."

민줄리, It refers to the idiom "to be like water off a duck's back", which means "without any immediate or lasting effect". As you may or may not know, water runs off the back of a duck because ducks have feathers coated with a special oil. The meeting was tough but to them it was like water off a duck's back. The meeting seemed to have no effect on them. metaphorical = representing a metaphor metaphor = a word or phrase that means one thing and is used for referring to another thing in order to emphasize their similar qualities