also: いせいのよい
expression, i-adjective (yoi/ii class)
high-spirited; cheerful; assertive; vigorous
Describes a person or their manner as lively, confident, and full of energy; often used for someone who is boldly cheerful or aggressively upbeat.
彼はいつも威勢のいい声で挨拶する。
He always greets people in a cheerful, vigorous voice.
威勢のいい掛け声とともに
The festival began with a lively, spirited shout.
Compound of 威勢 (いせい, 'spirit, vigor, authority') + の (possessive particle) + いい/よい ('good'). The phrase literally means 'of good spirit' and is used as a set expression.