adverb, noun, noun which may take the genitive case particle 'no'
in all sincerity; with one's whole heart; whole-hearted devotion
A yojijukugo expressing utmost sincerity and wholehearted effort, often used in formal or earnest contexts.
彼は誠心誠意、その問題に取り組んだ。
He tackled the problem with all his heart.
誠心誠意をもってお詫び申し上げます。
I offer my sincerest apologies.