(as|) slippery as (an eel|a snake)

used to refer to a deceitful and untrustworthy person who cannot be easily caught or exposed

The idiom "slippery as an eel" refers to the fact that eels have a slimy and slippery outer coating, which makes them difficult to hold or grasp. This idiom is used to describe someone who is evasive, elusive, or hard to pin down, especially when it comes to avoiding responsibilities, giving direct answers, or escaping from difficult situations.

Don't trust him; he's as slippery as a snake, always finding ways to avoid taking any real responsibility for his actions.
Don't trust him; he's as slippery as an eel, always finding ways to avoid taking any real responsibility for his actions.
The suspect in the crime is as slippery as an eel, making it challenging for the detectives to gather enough evidence.
With her quick wit and resourcefulness, she'll become as slippery as a snake in negotiations, securing the best deals for her company.