弟弟
dìdi · HSK 1
little brother
👦 弟弟 = younger brother + younger brother (reduplication for kin term)
In Mandarin, family terms often double up: 哥哥 older bro, 姐姐 older sis, 妹妹 younger sis, 弟弟 younger bro. The character 弟 alone means younger brother; doubling it makes the casual everyday word.