slippery as an eel - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.