means A many-headed serpent of Greek myth that grew two new heads for each one cut off; by extension, any problem that multiplies the more you try to eliminate it.

from From Greek 'Hydra,' the water-serpent of the Lerna swamp slain by Heracles, whose name comes from 'hydor,' water — the same root that swims through 'hydrant,' 'hydrogen,' and 'dehydrated.' The creature was at home in the marshes, and its name simply meant 'water-thing,' which is why the very word is soaked through with the sea and the spring.