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1. A tendency to speak maliciously. A: "He has such a serpent's tongue that I hate to be around him." B: "Of course you do—who wants to be verbally attacked all the time?"
2. A nickname for a fossilized shark's tooth. I think they call a shark's tooth a serpent's tongue because of how the root is forked.
See also: tongue
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.