late 14c., idroforbia, a symptom of rabies in man (sometimes used for the disease itself), from Late Latin hydrophobia (Celsius, 50 C.E.), from Greek hydrophobos "dreading water," from hydr-, stem of hydor "water" (see water, n.1) + phobos "dread, fear" (see phobia). So called because human sufferers show aversion to water and have difficulty swallowing it. In Old English as wæterfyrhtness.