reservoir (n.)

1680s, "a place where something tends to collect," originally figurative, from French réservoir "storehouse," from Old French reserver "set aside, withhold," from Latin reservare "keep back, save up; retain, preserve," from re- "back" (see re-) + servare "to keep, save, preserve, protect" (from PIE root *ser- (1) "to protect"). Specific meaning "artificial basin to collect and store a large body of water" is from 1705.