This is the 1749th most frequent Chinese word.
弟弟
Younger brother.
Here, '弟弟' refers directly to the speaker's younger brother, denoting a familial relationship.
我弟弟喜欢踢足球。
My younger brother enjoys playing football.
In this sentence, '弟弟' specifies the younger brother of the mentioned neighbor, using context to identify the individual.
她把玩具送给邻居家的弟弟。
She gave the toy to the neighbor's younger brother.
The plural form '弟弟们' implied by context signifies multiple younger brothers.