adverb
vigorously; with energy; assertively
Describes doing something with lively energy, confidence, or forcefulness; often used for speaking, acting, or moving in a spirited manner.
彼は威勢よく返事をした。
He replied vigorously.
選手たちは威勢よくグラウンドに飛び出した。
The players energetically ran out onto the field.
勢いよく emphasizes momentum and force, while 威勢よく adds a nuance of spirited confidence or assertiveness.
Compound of 威勢 (いせい, 'spirit, energy') and the adverbial form よく of the adjective よい ('good').