In its earliest appearances in English, in the 16th century, heyday was used as an interjection that expressed elation or wonder (similar to our word hey, from which it derives). Within a few decades, heyday was seeing use as a noun meaning "high spirits." This sense can be seen in Act III, scene 4 of Hamlet, when the Prince of Denmark tells his mother, "You cannot call it love; for at your age / The heyday in the blood is tame…." The word's second syllable is not thought to be borne of the modern word day (or any of its ancestors), but in the 18th century the syllable's resemblance to that word likely influenced the development of the now-familiar use referring to the period when one's achievement or popularity has reached its zenith.