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?")