This is the 13815th most frequent Korean word.
막내아들의
"Youngest son's" (possessive form).
In this sentence, '막내아들의' is used as a possessive to describe something (the hobby) belonging to the youngest son.
막내아들의 취미는 축구입니다.
The youngest son's hobby is soccer.
Here, '막내아들의' is used to indicate the ownership of the room by the youngest son.
나는 막내아들의 방에 들어갔다.
I entered the room of the youngest son.
In this sentence, '막내아들의' specifies the relation of the birthday to the youngest son.