heydayyoudaoicibaDictYouDict[heyday 词源字典]
heyday: [16] Etymologically, the -day of heyday has no connection with the English noun day, although it has come to resemble it over the centuries. Nor is hey- related to hay. Originally the word was heyda, an exclamation roughly equivalent to modern English hurrah. Probably it was just an extension of hey, modelled partly on Low German heida ‘hurrah’. Its earliest noun use (first recorded in the 1590s) was in the sense ‘state of exultation’; the influence of the day-like second syllable did not make itself felt until the mid-18th century, when the modern sense ‘period of greatest success’ began to emerge.
[heyday etymology, heyday origin, 英语词源]
heyday (n.)youdaoicibaDictYouDict
late 16c., alteration of heyda (1520s), exclamation of playfulness or surprise, something like Modern English hurrah, apparently an extended form of Middle English interjection hey or hei (see hey). Modern sense of "stage of greatest vigor" first recorded 1751, which altered the spelling on model of day, with which this word apparently has no etymological connection.