'acid-free' originates from English, specifically a compound of the words 'acid' and 'free', where 'acid' ultimately comes from Latin 'acidus' meaning 'sour' and 'free' comes from Old English 'frēo' meaning 'not in bondage' or 'without'.

'acid' changed from the Latin word 'acidus' (via Old French 'acide') and became the modern English 'acid'; 'free' evolved from Old English 'frēo' to 'free'; the descriptive compound 'acid free' in modern usage was later standardized with a hyphen as 'acid-free'.

Initially, the compound simply meant 'free from acid'; over time this basic meaning has remained but it acquired a specialized usage in conservation and manufacturing (especially for paper) to mean 'chemically neutral and long-lasting'.