wise younger brother; polite term for another's younger brother or junior
Archaic/literary pronoun used to address or refer to a younger brother or a junior with respect. Not used in modern everyday conversation; appears in historical texts or formal letters.
How are you faring, my wise younger brother?
手紙の書き出しに「賢弟」と記すことがあった。
In letters, one would sometimes write '賢弟' as the opening salutation.