The story
From pot + ash, because the substance was originally obtained by leaching ash in water and then boiling down the resulting solution in a pot. Compare Dutch potas, German Pottasche.
Etymology from Wiktionary, CC BY-SA 4.0.
potash
Today it means: The water-soluble part of wood ash, used for making soap and glass and as a fertilizer (chemically speaking, an impure form of potassium carbonate (K₂CO₃) mixed with other potassium salts). Full definition →