great debt of gratitude; great obligation
Refers to a profound sense of indebtedness for a significant favor or kindness received. Often used in formal or heartfelt expressions of thanks.
I owe him a great debt of gratitude, so I want to repay it over my lifetime.
You must not forget a great obligation.
Compound of 大 (dai, 'great') and 恩 (on, 'favor, obligation'). The reading is on'yomi, reflecting its Sino-Japanese origin.