Neologism
In psychiatry, the term neologism is used to describe the use of words that have meaning only to the person who uses them, independent of their common meaning. This tendency is considered normal in children. In adults, it can be a symptom of psychopathy  or a thought disorder, such as a psychotic mental illness, for example schizophrenia.  People with autism may also create neologisms. Additionally, use of neologisms may be related to aphasia acquired after brain damage resulting from a stroke or head injury.