noun, na-adjective
brimming with vitality; full of vigor; full of life
Describes a person or their state as being extremely energetic and lively. Often used as a な-adjective or noun.
彼はいつも元気旺盛で、周りを明るくする。
He is always brimming with vitality and brightens up those around him.
元気旺盛な子供たちが公園で遊んでいる。
Children full of life are playing in the park.
Compound of 元気 (genki, 'energy, vitality') and 旺盛 (ōsei, 'vigorous, abundant'). The exact historical derivation is uncertain, but the combination is a straightforward emphatic expression.