백골난망이 · 백골난망을
eternal gratitude, a kindness never forgotten even after death
Example
어려울 때 도와주신 은혜는 정말 백골난망입니다.
I will never forget how you helped me in hard times, even after I die.
Word parts
백 white+골 bone+난 hard+망 forget
Literally 'hard to forget even as white bones' — gratitude that outlasts death.
Good to know
Very formal and old-fashioned; used to thank someone deeply for a great favor.