In high spirits; full of vigor and enthusiasm. A four-character idiom often used in formal or written contexts.
新しい事業に意気軒昂で取り組んでいる。
They are tackling the new venture with high spirits.
入社したばかりの彼は意気軒昂としていたが、すぐに現実に直面した。
He was full of enthusiasm when he first joined the company, but soon faced reality.
古い小説では若者を意気軒昂と描く表現がよく見られる。
In older novels, it's common to depict young people as ebullient and high-spirited.