Used in reference to brittle lies, that's to say lies that when called out shatter quickly. A person may be called glass tongued, 'they have a glass tongue', or similar. The term originates during the Magic Enlightenment or Planar Catastrophe eras. One of the types of Extraplanars was a demon rumored to have a tongue made of molten glass. They'd weave strands of glass into lies but could be defeated by finding the solidified strands and shattering them.