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.
元気 is the common word for 'energetic' or 'in good spirits'. 元気旺盛 is a stronger, more emphatic compound meaning 'brimming with vitality'.
旺盛 means 'vigorous' or 'abundant' and is often used in compounds like 食欲旺盛 (hearty appetite). 元気旺盛 combines it with 元気 for extra emphasis on vitality.
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.