This is the 4773rd most frequent Japanese word.
次男
Second son.
次男 is used here to specify someone's birth order in the family.
彼は家族で次男です。
He is the second son in the family.
次男 refers to a specific member in a family during a special occasion.
次男の誕生日にはケーキを作ります。
I will make a cake for the second son's birthday.
次男 identifies a family member and their skill or characteristic.