"the act of straining or filtering through some porous material," 1610s, from Latin percolationem (nominative percolatio) "a straining through; the act of filtering," noun of action from past-participle stem of percolare "to strain through, filter," from per "through" (from PIE root *per- (1) "forward," hence "through") + colare "to strain," from colum "a strainer," which is of uncertain origin.
1620s, "to strain through" (transitive), a back-formation from percolation, or else from Latin percolatus, past participle of percolare "to strain through." Figurative sense by 1670s. Intransitive sense of "to pass through small interstices" is from 1680s. Related: Percolated; percolating.
"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."