Webster's Revised Unabridged Dictionary · 1913

Heyday

Heyday , interjection

[Compare German heida, or hei da, Dutch hei daar. Compare Hey, and There.]

An expression of frolic and exultation, and sometimes of wonder. — B. Jonson

Heyday , noun

[Probably for. high day. See High, and Day.]

The time of triumph and exultation; hence, joy, high spirits, frolicsomeness; wildness.
The heyday in the blood is tame. — Shakespeare
In the heyday of their victories. — J. H. Newman