'nontherapeutic' originates from Modern English, specifically the prefix 'non-' (a negating prefix meaning 'not') combined with 'therapeutic', which ultimately comes from Greek 'therapeuein'/'therapeia' meaning 'to attend, to cure'.

'therapeutic' came into English via Latin and French from Greek 'therapeuein'/'therapeia' and Middle/Modern English formed 'therapeutic'; the negating prefix 'non-' was attached in Modern English to form 'nontherapeutic'.

Initially 'therapeutic' meant 'relating to healing or treatment'; adding 'non-' produced the straightforward negated sense 'not therapeutic', and this basic opposed meaning has been maintained in modern usage.