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late 14c., originally a medical term, from Medieval Latin ephemera (febris) "(fever) lasting a day," from fem. of ephemerus, from Greek ephemeros "lasting only one day, short-lived," from epi "on" (see epi-) + hemerai, dative of hemera "day," from PIE *amer- "day."
Sense extended 17c. to short-lived insects and flowers; general sense of "thing of transitory existence" is first attested 1751. Cf. Greek ephemeroi "men," literally "creatures of a day."