'astigmatism-free' is a Modern English compound formed from 'astigmatism' (a medical noun) + 'free' (an adjective meaning 'without'). 'Astigmatism' ultimately derives from Greek elements where the prefix 'a-' meant 'without' and 'stigma' (στίγμα) meant 'point' or 'mark'; 'free' comes from Old English 'freo' meaning 'free, exempt'.

'astigmatism' entered English via New Latin/medical usage (from Greek 'astigmatismos' or 'astigmatos'), and combined with the English adjective 'free' in Modern English to form the compound adjective 'astigmatism-free'. 'Free' evolved from Old English 'freo' through Middle English into modern 'free'.

Initially the Greek-derived element conveyed the idea of 'without a point/mark'; in medical usage it came to denote the refractive condition called 'astigmatism'. Over time, compounding with 'free' produced the straightforward modern meaning 'without astigmatism'.