pleasant feeling; comfortable feeling
Refers to a sense of satisfaction, ease, or well-being. Often used in contexts like 快心の笑み (a satisfied smile) or 快心の作 (a work one is proud of).
He gave a satisfied smile.
This painting is a work he is proud of.
Compound of 快 (pleasant) and 心 (heart/mind). The exact historical derivation is uncertain, but the meaning follows naturally from the kanji.