Leach \Leach\, v. t. [imp. & p. p. {Leached}; p. pr. & vb. n. {Leaching}.] [Written also leech and letch.] 1. To remove the soluble constituents from by subjecting to the action of percolating water or other liquid; as, to leach ashes or coffee. 2. To dissolve out; -- often used with out; as, to leach out alkali from ashes.