근성이 · 근성을
grit, tenacity; (deep-rooted) nature, mentality
Example
그 선수는 끝까지 포기하지 않는 근성으로 유명해요.
That player is famous for his grit, never giving up.
Word parts
근 root+성 nature
Literally 'root nature' — deep-rooted character.
Good to know
Positive alone (grit), but negative in compounds like 노예 근성 'servile mentality'.