Origin: ME hei dai, full daylight, well on in the day < hei, high + dai, day
Origin: earlier heyda prob. < (or akin to) Ger & Dan heida, Du heidaar, hey there!: see hey
See heyday in American Heritage Dictionary 4
Origin: Perhaps alteration of heyda, exclamation of pleasure
Origin: , probably alteration of Middle English hey, hey.
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