heyday [E16th] From the early 16th century people shouted hey-day! to express joy, surprise, or some other intense emotion. It may have come from Low German heida! or heidi!, ‘hurrah!’. By the end of the same century heyday meant ‘a state of high spirits or passion’. Perhaps through a false association with *day, it began to refer to the period of a person’s or thing’s greatest success or activity in the mid 18th century.