'rehydrate' originates from Latin prefix 're-' and Greek element 'hydor' (via Neo-Latin/French 'hydrate'), specifically the element 'hydrate', where 're-' meant 'again' and 'hydor' (Greek) meant 'water'.

'hydor' (Greek) → element 'hydr-'/ 'hydro-' in Classical languages → Latin/French 'hydrate' → English 'hydrate' → modern English 'rehydrate' (re- + hydrate).

Initially it meant 'to add water' (literally to combine with water); over time it evolved into the current sense 'to restore water or fluids to something or someone (especially after drying or fluid loss)'.