also: しゅっこん
noun
old grudge; long-standing resentment
Refers to a grudge or resentment that has been held for a long time, often from past events. Somewhat literary or formal in tone.
彼は宿恨を晴らすために復讐を企てた。
He plotted revenge to settle an old grudge.
両家の間には長年の宿恨がある。
There is a long-standing resentment between the two families.
恨み is a more common and general term for grudge or resentment, while 宿恨 specifically emphasizes that the grudge is old or long-standing.
Compound of 宿 (しゅく, 'old, long-standing') and 恨 (こん, 'grudge, resentment'). The reading しゅっこん is a euphonic change.