名詞 heyday:

1. the period of greatest prosperity or productivity
   同義詞：flower, prime, peak, bloom, blossom, efflorescence, flush

1. "In his heyday, he was a great tennis player."
   ＂他年輕力壯時(shí)，是一位了不起的網(wǎng)球運(yùn)動(dòng)員。＂
2. Steam railways had their heyday in the 19th century.
   19世紀(jì)是蒸汽機(jī)車的全盛時(shí)期。

hey.day

n.（名詞）
The period of greatest popularity, success, or power; prime.
極盛時(shí)期：繁榮、成功或力量的最高潮；全盛時(shí)期

來源：
Perhaps alteration of heyda [exclamation of pleasure]
可能為 heyda的變化 [歡呼]
probably alteration of Middle English hey [hey]
可能為 中古英語 hey的變化 [嘿]