heyday

Etymology

Late 16th century, from earlier heyda (1520s), as exclamation – compare hey, hei. Sense “period of success, vigor” is a respelling as heyday based on unrelated day (as “period of time”) – compare day in the sun.

noun

1. A period of success, popularity, or power; prime.
   The early twentieth century was the heyday of the steam locomotive.
2. (archaic) An exultation of the spirits; gaiety; frolic.

intj

1. A lively greeting.
   "Heyday, Miss Morland!" said he. "What is the meaning of this? I thought you and I were to dance together." 1798, Jane Austen, Northanger Abbey
2. (obsolete) An expression of frolic and exultation, and sometimes of wonder.
   "Come follow me, my wags, and say, as I say. There's no riches but in rags; hey day, hey day, &c." 1600, Ben Jonson - Cynthia's Revels
   Bass. Heydey! good words, good words, I must beleeue'em And be a Coxcombe for my labor. 1633, John Ford, The Broken Heart, act IV, scene i, page 252