성실하게 (seong-si-ha-ge) – sincerely/earnestly
While this word has the meaning of “sincerely”, it can’t be used to sign off letters or messages like in English.
Note that this word is strictly an adverb and not an ending-note word.
그 여자는 일을 좋아하기 때문에 날마다 회사에 일찍 오고 성실하게 일해.
That lady comes to office early every day and does her work sincerely because she loves her job.
그 남자 성격이 좋은지도 모르겠는데 아까 그가 꽤 설실하게 말한 것 같아.
I don’t even know that man’s personality well but it seemed like he was talking quite sincerely just now.