noun
happy mood; euphoric mood
Describes a light, cheerful, almost giddy state of happiness. Often used in casual contexts.
今日は給料日だから、ルンルン気分で帰宅した。
Today was payday, so I went home in a happy mood.
彼女は新しいドレスを着て、ルンルン気分でパーティーに向かった。
Wearing her new dress, she headed to the party in a euphoric mood.
ウキウキ is a mimetic word for a light, excited, buoyant feeling, often used as an adverb (ウキウキする). ルンルン気分 is a noun phrase that specifically describes a happy, euphoric mood, often with a slightly dreamy or giddy nuance.
ルンルン is a mimetic word representing a light, happy humming or singing feeling. Combined with 気分 (mood), it forms a set phrase meaning a euphoric mood. The exact origin of ルンルン is uncertain, but it likely imitates a cheerful tune or sound.