Dongsaeng is a Korean word describing a family or friendship relationship. It is used to refer to one's younger sibling or a close friend that is younger than oneself.
"I promised my dongsaeng I'll treat her for dinner if she did well on her test."
"Dongsaeng, make me a cup of coffee."
(Although the above statement is a command, 'dongsaeng' acts as a form of endearment like 'little brother' or 'little sister'. In other words: "Lil' bro, how about a cup of coffee for your big bro?")
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