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.
He is always brimming with vitality and brightens up those around him.
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.