'Shì shū' is an honorific term used to respectfully address or refer to a close friend of one’s father—someone who is roughly the same age and social standing as one’s father, but not a blood relative. The character 世 (shì) literally means 'generation' or 'world', and here conveys the idea of shared generational status; 叔 (shū) means 'uncle', specifically the younger brother of one’s father. Together, they form a respectful title that signals both familiarity and deference, reflecting traditional Chinese values of hierarchy and relational etiquette.

This term is commonly used in formal introductions, family gatherings, or written correspondence (e.g., letters or wedding invitations). It is not used for distant acquaintances or casual friends—it implies longstanding trust and mutual respect between families. While less common among younger urban speakers today, it remains widely understood and appropriate in conservative, rural, or ceremonial contexts, especially when addressing elders or in traditional ceremonies like weddings or ancestral rites.