Definitions for "Castile soap"
A kind of fine, hard, white or mottled soap, made with olive oil and soda; also, a soap made in imitation of the above-described soap.
Originally made from olive oil, the term “Castile” now refers to any mild soap. Although modern Castile soaps are widely available, they can be very alkaline to the skin and hair.
a good hard soap made from olive oil and sodium hydroxide