The Chinese character 泗 has two meanings. One is a "name used for water," and the other is "snot." The character is a combination of "water," 氵(수) and the number "four," 四(사). Though I could understand why the character for water would be used in a character for "snot," I could not understand why "four" would be used, so I decided to research it. Here is what I found out.
Up until sometime about the "Warring States" period, the Chinese character for "four" was four stacked horizontal lines, but because it was often confused with the character for "three," 三(삼), the Chinese started using 四(사) to represent "four." Originally, 四 represented "breath coming out of a nose," but over time that meaning disappeared leaving only the meaning for "four." Therefore, if we think of 四 as a nose and combine it with water, 氵, we get "snot," 泗.