"wash or drain by percolation of water, treat by downward drainage," by 1660s in cookery, perhaps from a dialectal survival from Old English leccan "to moisten, water, wet, irrigate," which, under Norse influence, became leak (v.). The word was used 18c. in technological senses, such as leach-trough, a device used in salt-works in which corns of salt taken from brine were set to drain dry, after which they were called leach-brine. Related: Leached; leaching. Hence leach (n.) "a preparation made by leaching or straining" (1630s), in later use especially "a separation of lye or alkali in solution."