A noun meaning 'in a good mood / good humour' (similar to 'in high spirits').
今日は上機嫌で、ずっと笑っている。
He's in a good mood today and keeps smiling.
上機嫌なときに仕事を頼むと受け入れてもらいやすい。
If you ask when someone is in a good mood, they're more likely to agree.
会議のあとで上機嫌になっていた。
He was in high spirits after the meeting.