정성 [ jeongseong ]
Among so many meanings of 정성, the one that is used more often in conversation among Koreans is ‘the faithful and true heart to do all the effort possible‘.
In this case, a verb 다하다 [ dahada ] is often used together, which means ‘to fulfill’, ‘to exhuast’, or ‘to do everything possible’. Another is 지극하다 [ jigeukada ], which means ‘to be extreme’, or ‘to be exceeding’, or 가득하다 [ gadeukada ], which means ‘to be full’.
정성을 다하겠습니다. (I will do my best with all my heart.)
정성이 가득한 식사 감사합니다. (Thank you so much for the great meal that you prepared for us.)
철수는 여자친구에게 지극정성이다. (Cheolsu is really good to his girlfriend and it seems like he will do everything for her.)