Merriam-Webster Collegiate Dictionary - lye

 noun  Etymology: Middle English, from Old English leag; akin to Old High German louga ~, Latin lavare, lavere to wash, Greek louein  Date: before 12th century  1. a strong alkaline liquor rich in potassium carbonate leached from wood ashes and used especially in making soap and for washing; broadly a strong alkaline solution (as of sodium hydroxide or potassium hydroxide)  2. a solid caustic (as sodium hydroxide)