hard soap
/ˌhɑːd ˈsəʊp/noun
Originally: †soap that is solid rather than liquid (obsolete ). In later use: soap that is hard and does not readily dissolve, typically made with sodium salts rather than potassium ones.
Origin
Late Middle English; earliest use found in Guy de Chauliac's Grande Chirurgie. From hard + soap, originally after post-classical Latin sapo durus.