'decontaminate' originates from Latin elements: the prefix 'de-' (from Latin) and the Late Latin word 'contaminare', where 'de-' meant 'off/away/reverse' and 'contaminare' meant 'to defile, pollute'.

'contaminare' passed into Old French and Middle English as words like 'contaminer'/'contaminate' and eventually became the modern English 'contaminate'; the verb 'decontaminate' was formed in English by adding the prefix 'de-' to mean removal of contamination.

Initially 'contaminare' meant 'to pollute or defile'; with the addition of 'de-' the word evolved into 'decontaminate', meaning 'to remove pollution or contamination'.