Saponaceous - sap·o·na·ceous [sap-uh-ney-shuhs]
Soapy, slippery, evasive, having the consistency or likeness of soap or slipperiness. Can be used to describe someone or something.
From New Latin Saponaceus, from Latin sapon (soap)
Example: Try as he might to think of a way out of it, the saponaceous little bastard was right - the contract was air-tight. But on the plus side, John could do what he liked with his newly won soul - Freddy was an Atheist!