Pronunciation: (un-dOO'ing),[key] —n. 1. the reversing of what has been done; annulling.
2. a bringing to destruction, ruin, or disaster.
3. a cause of destruction or ruin.
4. the act of unfastening or loosing.
5. Psychiatry.an unconscious defense mechanism through which an attempt is made to reverse a psychologically unacceptable act by doing its opposite, usually repetitiously, in order to relieve anxiety.