cheerfully; happily; in good humor
Describes doing something in a cheerful, willing, or good-natured manner. Often used when someone acts without complaint or with a pleasant attitude.
彼は機嫌よく手伝ってくれた。
He cheerfully helped me.
機嫌よく歌を歌っている。
She is singing happily.
上機嫌 means 'in high spirits' or 'very good mood', a stronger and more specific state than the general cheerfulness of 機嫌よく.
Standard kanji spelling for the adverbial phrase.
Alternate kanji spelling using 良く instead of よく.
Formed from the noun 機嫌 (mood, temper) and the adverbial form of 良い (よい), meaning 'in a good mood' or 'cheerfully'.