"substance obtained by leaching wood-ashes, evaporating the solution obtained, and calcining the residuum; one of the fixed alkalis," 1751, earlier -pot-ashes (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.