late 14c., originally a medical term, from Medieval Latin ephemera (febris) "(fever) lasting a day," from fem. of ephemerus, from Greek ephemeros "daily, for the day," also "lasting or living only one day, short-lived," from epi "on" (see epi-) + hēmerai, dative of hēmera "day," from PIE *Hehmer "day." Sense extended 17c. to short-lived insects (Modern Latin ephemera musca) and flowers; general sense of "thing of transitory existence" is first attested 1751. Compare Greek ephemeroi "men," literally "creatures of a day."