with gusto; enthusiastically; energetically
Describes doing something in a lively, spirited, or wholehearted manner, often with a sense of cheerful vigor.
He downed his beer with gusto.
Let's sing a song energetically.
Alternate spelling using 良く; less common but still seen.
From 景気 (けいき, 'spirits; liveliness') + adverbial よく (continuative of よい, 'good'). Literally 'in a good-spirited way'.