Saponaceous is an adjective that means slippery.

The Latin word sapon (SOP on) means ‘soap.’ In time, this word evolved into our word of the day which means ‘soap.’ It may refer literally to something made of soap or, in a more figurative sense, it may mean ‘slippery’ as in ‘elusive.’ For example: When it comes to tracking him down, James can be a bit saponaceous. Just when you think you’ve found him, he eludes you once again.