late 14c., originally a medical term, from M.L. ephemera (febris) "(fever) lasting a day," from fem. of ephemerus, from Gk. ephemeros "lasting only one day," from epi "on" + hemerai, dat. of hemera "day," from PIE *amer- "day." Sense extended to short-lived insects and flowers; general sense of "transitory" is first attested 1630s.
1620s, from Gk. (zoon) ephemeron, neut. of ephemeros (see ephemera).