slippery as an eel

To be slippery as an eel means that you are very devious and untrustworthy and often impossible to catch. Eels are extremely slippery and silk creatures making it quite easy for them to escape people trying to catch them in nets or by hand by reaching into a tank. To be slippery as an eel means this person is not only difficult to catch but also a devious and rather untrustworthy type of person.