'inflexible' originates from Latin, specifically the word 'inflexibilis,' where 'in-' meant 'not' and 'flexibilis' meant 'able to be bent' (from 'flectere', to bend).

'inflexibilis' passed into Old French and Middle English (as forms like 'inflexible'), and eventually became the modern English adjective 'inflexible.'

Initially, it meant 'not able to be bent' (physically rigid); over time it extended to mean 'not willing to change' or 'unwilling to compromise' in behavior or opinion.