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1835, genus name of a freshwater polyp, from Greek Hydra, many-headed Lernaean water serpent slain by Hercules (this sense is attested in English from late 14c.), from hydor (genitive hydatos) "water" (see water (n.1)); related to Sanskrit udrah "aquatic animal" and Old English ottur "otter." Used figuratively for "any multiplicity of evils" [Johnson]. The fabulous beast's heads were said to grown back double when cut off, and the sea creature is said to be so called for its regenerative capabilities.
Plural hydras or hydrae (hī'drē)
See under hydroid.