na-adjective
passionate; hot-blooded; full of vitality
Describes a person, especially a young one, who is energetic, impulsive, and driven by strong emotions or desires.
彼は血気盛んな若者だ。
He is a hot-blooded young man.
血気盛んなあまり、無茶をしてしまうこともある。
Being so full of vitality, he sometimes does reckless things.
熱血 is also 'hot-blooded' but often used for enthusiastic dedication to a cause, whereas 血気盛ん focuses more on raw vitality and rashness.
Compound of 血気 (けっき, 'blood and spirit; vigor') and 盛ん (さかん, 'prosperous; vigorous'). The exact historical derivation is uncertain, but the combination naturally yields the meaning 'full of youthful vigor'.