The Collaborative International Dictionary of English v.0.48:

Saponaceous \Sap`o*na"ceous\, a. [L. sapo, -onis, soap, of
   Teutonic origin, and akin to E. soap. See Soap.]
   Resembling soap; having the qualities of soap; soapy.
   [1913 Webster]

   Note: Saponaceous bodies are compounds of an acid and a base,
         and are in reality a kind of salt.
         [1913 Webster]

WordNet (r) 3.0 (2006):

saponaceous
    adj 1: resembling or having the qualities of soap; "a soapy
           consistency" [syn: saponaceous, soapy]