na-adjective, noun which may take the genitive case particle 'no', adverb, noun
full of energy; brimming with vitality; very healthy
Describes a person or living thing that is visibly energetic, lively, and in good health. Often used in everyday speech to emphasize high spirits.
子供たちは元気いっぱいで公園を走り回っている。
The children are running around the park full of energy.
今日は元気いっぱいだね。
You're brimming with energy today, aren't you?
Compound of 元気 (genki, 'health, energy') + いっぱい (ippai, 'full'). The phrase is straightforward and modern.