Definition of Castile soap in English:

Castile soap

noun

  • Fine, hard white or mottled soap made with olive oil and sodium hydroxide.

    • ‘Our soaps are made using a traditional Castile soap formula from the 19th century.’
    • ‘This produced what was probably the first white hard soap: Jabon de Castilla, or Castile soap, also known to pharmacists as Sapo hispaniensis or Sapo castilliensis.’
    • ‘Because of its mildness, Castile soap became the soap of choice for babies and those with sensitive skin.’
    • ‘Often based on pure Castile soap, natural shampoos are usually combined with high-quality herbs and plant oils that leave your hair and scalp healthier.’

Origin

Late Middle English: named after Castile in Spain, where it was originally made.