Etymology
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leach (v.)

"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."

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leachate (n.)

"that which is formed by or results from leaching," 1920, from leach + -ate (1). The older noun was simply leach.

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letch (n.)
"craving, longing, strong desire," 1796 [Grose], perhaps a back-formation from lecher, or deformed from a figurative use of latch (v.) in a secondary sense of "grasp, grasp on to." Or perhaps from letch (v.), a variant of leach.
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