noun
great debt of gratitude; immense obligation
Formal, somewhat literary term for a profound favor or kindness that one can never fully repay. Often used in expressions of deep gratitude, such as in letters or speeches. Compare with 大恩 (たいおん), which is more common.
See also: 大恩
先生の洪恩に報いることは到底できません。
I can never repay the great kindness of my teacher.
このたびの洪恩、一生忘れません。
I will never forget this immense favor for the rest of my life.
大恩 is the more common and slightly less formal term for 'great obligation' or 'deep gratitude'. 洪恩 is more literary and emphasizes an overwhelming, almost boundless favor.
Compound of 洪 (vast, flood) or 鴻 (large bird, great) and 恩 (grace, kindness). The exact historical derivation is uncertain, but both kanji choices reinforce the idea of an immense, overwhelming favor.