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.
The players faced the match in high spirits.
He's been even more full of vigor since leaving the hospital.
The new employees are full of enthusiasm, but they still lack experience.
The players tackled practice with high spirits.
A group of spirited young people gathered at the venue.
活発 describes lively, active behavior in general, whereas 意気盛ん specifically highlights strong morale or enthusiasm.
意気揚々 describes a triumphant, proud mood usually after success, while 意気盛ん simply means being full of vigor or spirit in general.
意気消沈 means dejected or downhearted, the opposite state from 意気盛ん, which means full of spirit and vigor.
意気地 means 'backbone' or 'guts' (as in 意気地なし, 'coward'), a different concept from 意気盛ん's 'full of vigor', despite sharing the kanji 意気.
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.