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Phrase(s): *slippery as an eel
devious and untrustworthy, but impossible to catch. (*Also: as ~.) • Don’t sign a lease with that landlord; I think he’s as slippery as an eel. • The con artist was slippery as an eel. Although he defrauded many people, he never went to prison.
McGrawhill's American Idioms And Phrasal Verbs
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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.
American Heritage Idioms