noun
my younger brother
Humble term used to refer to one's own younger brother when speaking to someone outside the family. Shows modesty about one's own family member.
愚弟がお世話になっております。
My younger brother is in your care.
愚弟はまだ学生でございます。
My younger brother is still a student.
noun
foolish younger brother
Literally 'foolish younger brother', but in practice this is the same humble usage as sense 1; the 'foolish' nuance is part of the self-deprecating humble expression rather than a separate meaning. Rarely used to actually call one's brother foolish.
愚弟の不行き届きでご迷惑をおかけしました。
I apologize for the trouble caused by my foolish younger brother's oversight.