heyday

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). Around the same time, "heyday" saw use as a noun meaning "high spirits." (This sense can be seen in Act III, Scene IV 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….") It wasn’t until the 18th century that English speakers, perhaps interpreting the "day" of the second syllable to mean "a time or period," began using "heyday" to refer to the period when one’s achievement or popularity has reached its zenith.