Hi,
I expect that the patient was given this diagnosis by a practitioner outside of the conventional medical model, or that a more specific term was used by a medical person and then a descriptive lay term the patient could more easily understand was added (obviously did not work!). It would not be the first time a patient muddled information provided by a therapist.
I do not believe that a search of the literature would be useful to define the term, as it is likely to be an umbrella term without specificity. Many conditions could be referred to inaccurately as a 'deep arthritis'. It would be best to accept the most recent assessment findings, and re educate the patient once the assessment is confirmed.
Loose diagnostic terminology can be used by psuedo trained 'therapists' to mislead a patient or to cover their own lack of knowledge.
Hope this helps.
MrPhysio