The word 表弟 is formed by combining two characters, each contributing a specific relational idea. The first character, 表 (biǎo), historically denotes relationships considered "outside" the primary paternal lineage, typically referring to connections through one's mother or married female relatives. The second character, 弟 (dì), directly means "younger brother". Therefore, the compound 表弟 is constructed to precisely indicate a specific male cousin: a younger brother from that external maternal branch of the family.