slippery as an eel
Elusive, devious, as in When it comes to talking about his investments, Jim's slippery as an eel. This simile, first recorded about 1412, alludes to the eel's skin, which has tiny scales and is quite slippery when wet.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.