verb

• (transitive) To cause (someone) to unlearn; to make (someone) forget something they have been taught, or recognize it as erroneous, etc.
  "The heart of man has been long mistaught, and, I fear, it will require something that shall come closer home than legal enactments to unteach it. And nothing but example can teach it anew. It will never learn to love instead of to hate, […]"
• (transitive) To cause (something previously learned) to be forgotten, or recognized as an error, etc.
  ""I did say that the mass was the Romish name for the sacrament." Upon which the rector "beat his head like a man distracted;" and declared that he should into the school and unteach that also."