noun
happy-go-lucky person; easy-going person; optimist
Describes a person who is carefree, relaxed, and not easily worried. Often used in a mildly positive or teasing way.
彼はのんき者で、いつも笑っている。
He is a happy-go-lucky person and is always smiling.
のんき者だから、試験前でも平気だ。
Because she's an easy-going person, she's calm even before an exam.
のんき is an ateji reading of 呑気, originally from Chinese. 者 is a suffix meaning 'person'. The exact historical derivation of のんき is uncertain.