'heterodox' originates from Greek, specifically the word 'heterodoxos', where the prefix 'hetero-' meant 'other, different' and 'doxa' meant 'opinion' or 'belief'.

'heterodoxos' passed into Medieval Latin as 'heterodoxus' and then into Middle English (via learned Latin usage) as 'heterodox', becoming the modern English adjective 'heterodox'.

Initially it meant 'of different opinion' (literally 'different belief'), and over time it came to mean more specifically 'not conforming to accepted or orthodox doctrines' (often with a religious or doctrinal implication).