‘姑妹’ (gū mèi) is a kinship term meaning ‘younger sister of one’s father’ — that is, the father’s younger sister. It combines 姑 (gū), which on its own means ‘father’s sister’, and 妹 (mèi), meaning ‘younger sister’. While 姑 alone can refer to any paternal aunt (older or younger), adding 妹 specifies she is younger than one’s father. This precise distinction reflects Chinese’s rich system of familial terms, where age relative to the parent matters more than in English.

The term is used formally or respectfully when speaking about or addressing this relative, especially in family gatherings, introductions, or written contexts like letters or genealogies. It’s less common in casual daily speech among younger generations, who may use simpler terms like ‘Auntie’ + name, but remains essential for understanding traditional family structures and formal communication. Note that it does not refer to a cousin — ‘cousin’ would be 表妹 (biǎo mèi) or 堂妹 (táng mèi), depending on maternal/paternal line.