1802, "compound of water and another chemical," from French hydrate, coined c. 1800 by French chemist Joseph-Louis Proust (1754-1826) from Greek hydr-, stem of hydor "water" (see water (n.1)).[hydrate etymology, hydrate origin, 英语词源]
1850, "to form a hydrate;" 1947 as "to restore moisture;" from Greek hydr-, stem of hydor "water" (see water (n.1)) + -ate (2). Related: Hydrated; hydrating.