Psychopathological functioning is a term that is utilized in the Freudian school of ego psychology. Specifically, it refers to the maladaptive behavior that can result from an individual's failure to successfully resolve ego conflicts at the various stages of development.
Freud believed that specific types of maladaptive behavior could be traced to failures to resolve the ego conflicts at one of the five stages of sexual development that he had posited. These stages (known as the oral, anal, phallic, latent and genital phases) occur at predictable periods of life and are based on the anatomical part that is most important to that stage.
For example, the oral stage (birth to 1 year) is when the primary source of physical pleasure is the mouth due to the desire to breast feed and the habit of using the mouth to explore the world.
An example would be a failure to resolve a conflict during the oral stage could create an immature personality due to having received too much or too little gratification at that stage of life. For instance, it has been hypothesized that under this theory that smoking is a behavior related to the oral stage of development.
Likewise, Freud believed that many other difficulties could be traced to problems during the other four of these phases.