The abbreviation is used in ICD-9 coding. Best to avoid in actual charting; “unknown etiology” is more appropriate. Reserve “NOS” for when you want to convey a vague symptom that you don’t intend to pursue diagnostically. In general conversation, the epithet “P.D.N.O.S.” works nicely for certain types of people [“personality disorder NOS”]. More polite than other expressions we might mobilize.