reservoir (n.) Look up reservoir at Dictionary.com
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.