'non-' originates from Latin 'non' meaning 'not'. 'esterified' is formed from 'ester' + the verb-forming suffix '-ify' (from Latin facere 'to make') with the past participle suffix '-ed', so 'esterified' means 'made into an ester'.

'ester' entered chemical vocabulary in the early 19th century to name compounds derived from acids and alcohols; adding '-ify' produced 'esterify' (to make into an ester), whose past participle 'esterified' was later combined with the negative prefix 'non-' to form 'non-esterified'.

Initially, 'esterify' meant 'to convert into an ester'; over time, especially in biochemical contexts, 'non-esterified' came to be used as a concise descriptor for molecules (notably fatty acids) that remain unconverted—i.e., 'free' fatty acids.