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
I can never repay the great kindness of my teacher.
I will never forget this immense favor for the rest of my life.
Standard kanji form; 洪 means 'vast' or 'flood', emphasizing magnitude of the favor.
Variant using 鴻 (large bird/swan), also conveying great scale; less common but still seen.
A compound of 洪 “vast, immense” and 恩 “favor, grace,” denoting an immeasurably great kindness or obligation.