Na-adjective (keiyodoshi), Noun
easy; easygoing; carefree; happy-go-lucky; optimistic; leisurely; careless; thoughtless
Usually written using kana alone
お前は当事者じゃないから、そんなのんきなこと言ってられるんだよ。You wouldn't be so casual about it if you were directly involved.
Also written 暢気【のんき】、暖気【のんき】