Silver
tongue is an expression used to describe a person who has a clever way with words.
Add some clever sarcasm, or sleazy lies to this person's attitude, and there you have your silver tongued
devil.
I honestly can't believe I'm making a bible reference, but this is what Google gave up for silver tongued
devil:
"When Satan with his silver-tongue approached Eve, using the serpent as a mouthpiece, note that the very first thing he did was to question the veracity of
God’s spoken word. Paraphrasing, Satan said to Eve, ‘Has
God said you shall surely die if you disobey?
God has not told you the
truth. You will not surely die, for God knows, though he has not told you, that in the day you eat the forbidden fruit your eyes will be opened and you yourselves shall be as
God, and you will know good and evil, that is, be able to decide for yourselves what is right and what is wrong."(Gen. 3:1-5)
