An alternate or archaic spelling of heyday; a time of great success, popularity, or youthful vigor.
A variant spelling of heyday, from hey (an exclamation) + day, reflecting older English orthographic practices where spelling was less standardized.
Spelling variations like 'heydey' versus 'heyday' show how English spelling was once a free-for-all—Shakespeare himself spelled his name different ways, so standardized spelling is actually a pretty recent invention.
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