www.alphadictionary.com

This is so commonly associated with liar, as in he is such a pathological liar. The root is disease, so that a pathologist is NOT a medical doctor who lies, but one who studies disease. Someone who lies frequently it appears does so as if diseased.

Somewhat touched upon in the discussion of sociopath. I guess what we can do is connect the dots of sociopath sociology psychopath psychology and pathology. Not entirely satisfactory, but perhaps of use to someone out there.