"compound of water and another chemical," 1802, from French hydrate, coined c. 1800 by French chemist Joseph-Louis Proust (1754-1826) from Greek hydr-, stem of hydor "water," from suffixed form of PIE root *wed- (1) "water; wet."). Also formerly applied to compounds formed on the same type as H2O.
1812 (implied in hydrated), "to form a hydrate, combine chemically with water," from hydrate (n.), perhaps modeled on French hydrater. From 1947 as "to restore moisture;" from Greek hydr-, stem of hydor "water" (from suffixed form of PIE root *wed- (1) "water; wet") + -ate (2). Related: Hydrating.