The concept of 表弟 (biǎodì) is a window into the structure of traditional Chinese society. The key distinction is between 表 (biǎo) and 堂 (táng) relatives.
This linguistic split highlights the patrilineal emphasis in traditional Chinese culture, where descent and inheritance were traced through the male line. The 堂 family was the “main hall” or central clan, while the 表 family, though still close, was considered external. In Western cultures, the word “cousin” is a broad, catch-all term. You might clarify by saying “my cousin on my mom's side,” but this is an added description. In Chinese, this distinction is fundamental and embedded directly into the noun itself. While modern urban families may not place as much emphasis on the patrilineal clan, these terms remain the standard and correct way to address and refer to one's relatives.
表弟 (biǎodì) is a common, everyday term used to refer to or address a specific family member.