Castile soapCastile soap
a fine, mild, hard soap prepared from olive oil and sodium hydroxide
Origin of Castile soapMiddle English Castell sope, after Castile, where first made
also cas·tile soap
A soap made with vegetable oil, especially olive oil, and sodium hydroxide.
Origin of Castile soapAfter Castile.
(countable and uncountable, plural Castile soaps)
- A kind of fine, hard, white or mottled soap, originally made with olive oil.
From Castile in Spain, from which it originally came.