1835, genus name of a freshwater polyp, from Gk. Hydra
, many-headed Lernaean water serpent slain by Hercules (this sense is attested in Eng. from late 14c.), from hydor
) "water" (see water
(n.1)); related to Skt. udrah
"aquatic animal" and O.E. ottur
"otter." Used figuratively for "any multiplicity of evils" [Johnson].