'heresy-monger' originates from English, specifically the words 'heresy' and 'monger', where 'heresy' ultimately comes from Greek 'haíresis' via Latin 'haeresis' and Old French 'heresie' meaning 'choice, school of thought', and 'monger' comes from Old English 'mangere' meaning 'dealer, seller'.

'heresy' changed from Greek 'haíresis' to Latin 'haeresis', then Old French 'heresie' and Middle English 'heresie', eventually becoming modern English 'heresy'. 'monger' changed from Old English 'mangere' to Middle English 'monger' meaning 'dealer' and was later used metaphorically in compounds (e.g. 'fishmonger'); the compound 'heresy-monger' was formed in Modern English by combining these elements to mean someone who 'sells' or promotes heresy.

Initially, 'heresy' referred to a 'choice' or sect and 'monger' to a 'seller' or dealer; combined they originally suggested a literal 'seller of heresy' and over time the term has been used metaphorically for anyone who promotes or spreads heterodox beliefs.