(with reference to a soluble chemical or mineral) drain away from soil, ash, or similar material by the action of percolating liquid, especially rainwater: [with object]:the nutrient is quickly leached away [no obj]:pesticides that leach into rivers
[with object] subject (soil, ash, etc.) to a leaching process.
Old Englishleccan 'to water', of West Germanic origin. The current sense dates from the mid 19th century