Example sentences
dì弟di弟jīn今nián年shàng上xiǎo小xué学。
My younger brother starts primary school this year.
dì弟di弟hěn很tiáo调pí皮。
My little brother is naughty.
About this word
弟 is younger brother, and the word is doubled to 弟弟. Chinese kinship terms almost all do this for the everyday form: 哥哥, 姐姐, 妹妹, 爸爸, 妈妈. Reduplication makes a single syllable easier to say and marks closeness, so 弟弟 is how you actually address or mention your younger brother, while a bare 弟 survives mainly in compounds like 兄弟 (brothers). The second syllable goes light: dìdi. There is no plural change, so 我的弟弟 can mean my brother or my brothers.
The story behind the character
弟
弟 did not start out meaning little brother. The oldest forms show a rope wound around a pole, and Xu Shen defined it as 韋束之次弟也, the order things fall into when bound. Sequence, rank, order. A younger brother is simply the next one in line. Chinese later built 第 with 竹 on top to carry the "order" sense and left 弟 meaning the younger brother.