also: いせいがよい
expression, i-adjective (yoi/ii class)
vigorous; energetic; assertive; cheerful
Describes a person, attitude, or manner that is full of energy, confidence, and spirit; often implies a lively, forceful, or high-spirited demeanor.
彼はいつも威勢がいい。
He is always full of energy and confidence.
威勢のいい掛け声で仕事を始めた。
They started work with a vigorous shout.
Compound of 威勢 (いせい, 'spirit, energy') + が + いい/よい ('good'). The phrase literally means 'spirit is good' and is used idiomatically to describe a vigorous or assertive manner.