"弟弟" 的英语是 "younger brother"。
其来源是英语中 "younger" 表示 "年龄较小的"，"brother" 表示 "兄弟"。
例句：
My younger brother is six years old. (我弟弟六岁。)
I helped my younger brother with his homework. (我帮助我弟弟做作业。)
My younger brother is very talented at playing the piano. (我弟弟非常擅长弹钢琴。)