"vegetable alkali; substance obtained originally by leaching wood-ashes and evaporating the solution obtained in a large iron pot or pan; one of the fixed alkalis," 1751, earlier pot-ash (1640s), a loan-translation of older Dutch potaschen, literally "pot ashes" (16c.); see pot (n.1) + ash (n.1).
So called because it was originally obtained by soaking wood ashes in water and evaporating the mixture in an iron pot. Compare German Pottasche, Danish potaske, Swedish pottaska, all also from Dutch. See also potassium. French potasse (1570s), Italian potassa are Germanic loan-words. The original plural was pot-ashes.