'pathological' originates from Medieval Latin/Modern Latin, specifically the word 'pathologicus', which itself comes from Greek elements where 'path-' (from Greek 'pathos') meant 'suffering, disease' and '-logicus' (from Greek 'logikos'/'-logia') meant 'study' or 'reason'.

'pathological' changed from Medieval Latin 'pathologicus' and Late Latin 'pathologicus' into Middle French/English forms and eventually became the modern English adjective 'pathological' by adding the English adjectival ending '-ical' to 'pathology'.

Initially, it meant 'relating to the study of disease' or 'of disease origin'; over time it has broadened to include 'relating to disease or abnormality' and metaphorically 'extremely compulsive or obsessive' as in 'pathological liar'.