弟弟 (dì di) is a noun meaning 'younger brother'—a male sibling who is younger than the speaker. The character 弟 (dì) on its own means 'younger brother' and is also used in formal or written contexts (e.g., 弟子 'disciple'). When reduplicated as 弟弟, it becomes the common, affectionate, and colloquial term used in everyday speech to refer to one’s own younger brother or someone else’s. Unlike the standalone form, the reduplicated version carries warmth and familiarity, and is rarely used for strangers without context.

This word follows a frequent Chinese pattern where reduplication of kinship terms softens tone and signals closeness or endearment (e.g., 哥哥 gē ge 'older brother', 妹妹 mèi mei 'younger sister'). It is not used for older brothers (that’s 哥哥 or 兄弟), nor does it imply age difference beyond being younger—it could refer to a brother just months younger or decades younger.