name of the many-headed Lernaean water serpent slain by Herakles in Greek mythology, late 14c., idre, from Greek Hydra, fem. of hydros "water-snake," from hydor "water" (from suffixed form of PIE root *wed- (1) "water; wet").
The word is etymologically related to Sanskrit udrah "aquatic animal" and Old English ottur (see otter). Used figuratively for "any multiplicity of evils" [Johnson]. The fabulous beast's heads were said to grown back double when cut off. As a constellation (see Hydrus), usually identified as the monster Herakles slew, from mid-15c. As the genus name of a freshwater polyp from 1798; said to have been so called by Linnaeus for its regenerative capabilities.