'not-affected' is a compound formed from the adverb 'not' and the past participle 'affected'. 'Not' comes from Old English 'nōt' (or 'ne + ōt'), meaning 'no' or 'not', and 'affected' is the past participle of 'affect', which derives ultimately from Latin 'afficere' (via Old French and Middle English).

'affected' comes from Latin 'afficere' ('affect-') which passed into Old French as forms like 'affecter' and into Middle English as 'affecten/affected'; combining it with the negator 'not' produced the straightforward compound phrase 'not affected' in Modern English to indicate the absence of effect or influence.

Initially, 'affected' literally meant 'influenced' or 'treated' (from Latin), but it later also gained a sense of 'artificial' or 'pretentious'. The compound 'not-affected' has therefore been used in both senses: 'not influenced' and 'not pretentious' (i.e., natural).