na-adjective, noun
in high spirits; full of vigor; spirited
Describes a person or group that is energetic, enthusiastic, and showing strong morale. Often used in formal or written contexts.
選手たちは意気盛んに試合に臨んだ。
The players faced the match in high spirits.
新入社員は意気盛んだが、まだ経験が足りない。
The new employees are full of enthusiasm, but they still lack experience.
元気 is a more common, everyday word for 'energetic' or 'healthy', whereas 意気盛ん is more formal and emphasizes high morale or fighting spirit.
Compound of 意気 (spirit, heart) and 盛ん (prosperous, vigorous). The exact historical derivation is uncertain, but the combination naturally conveys a state of high spirits.