Wiktionary
a. 1 (context simile colloquial English) So slippery that it is almost impossible to hold with one's hands 2 (context idiomatic simile colloquial English) (of a person) So crafty, or cunning that they cannot be caught by the police, although it is known that they are acting illegally.
Usage examples of "slippery as an eel".
A shrill cry of fright startled his ears, and a wiry little body, slippery as an eel, fought to elude his grasp and narrowly failed.
He tried to throw her off, but she was nimble and slippery as an eel with the coating of mud.
Ramses was as slippery as an eel and unrestrained by gentlemanly scruples.
I'll keep an eye on him until Hesach's ready-he's slippery as an eel.
The rock was like glass, and in places coated with a thick beard of algae, slippery as an eel's skin.
As slippery as an eel she wormed out from under him and flew at the door to the saloon.
The man they wanted was a New Zealand-born heroin czar, sought in a dozen countries but slippery as an eel.