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1540s, a medical word for "excess of body fluid," from Late Latin plethora, from Greek plethore "fullness," from plethein "be full" (see pleio-). Figurative meaning "too-muchness, overfullness in any respect" is first recorded 1700. Related: Plethoric.
plethora pleth·o·ra (plěth'ər-ə)
An excess of blood in the circulatory system or in one organ or area.
An excess of any of the body fluids.