heyday/ˈheɪdeɪ/EtymologyLate 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.heyday means A lively greeting. Lexicurio rates it Distinctive — a strength score of 69 out of 100.intjA lively greeting.“"Heyday, Miss Morland!" said he. "What is the meaning of this? I thought you and I were to dance together."”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."”nounA period of success, popularity, or power; prime.“The early twentieth century was the heyday of the steam locomotive.”An exultation of the spirits; gaiety; frolic.