/ɪnˈflɛksɪbəl/
[adjective] Describes someone or something that is unwilling or unable to change, bend, or adapt. An inflexible person refuses to accept new ideas or compromise, while an inflexible material cannot be physically bent without breaking.
The word 'inflexible' comes from a Latin root meaning to bend — so being inflexible literally means your mind cannot bend.
To understand 'inflexible', you have to start with bending. The Latin verb 'flectere' meant to bend, to curve, or to turn — and it was one of those wonderfully physical words that the Romans applied equally to the bending of a branch, the turning of a road, and the changing of a person's mind. From 'flectere' came the adjective 'flexibilis', meaning capable of being bent, adaptable, or yielding. Add the negative prefix 'in-' — which functions in Latin just as it does in English, reversing or negating what follows — and you get 'inflexibilis': that which cannot be bent, that which will not yield.
The word arrived in English in the 14th century, borrowed directly from Latin, as so many abstract and philosophical terms were during the great intellectual flowering of the medieval and early Renaissance periods. English writers of this era were reaching for words to describe qualities of mind and character, and they found Latin a rich storehouse. 'Inflexible' came in already carrying both its physical meaning — a material that could not be bent — and its moral one — a will or principle that could not be moved.
The root 'flectere' has been remarkably productive in English. Following the chain of transmission, Latin 'flectere' gave us 'flexus', its past participle, which became the foundation for 'flexible' (borrowed directly into English in the 15th century, meaning 'capable of bending'). Latin 'reflectere', meaning to bend back, passed through Old French as 'reflecter' and entered English as 'reflect'. Latin 'deflectere', meaning to bend away, became English 'deflect'. Even 'genuflect' — to bend the knee — comes from the same root, combining Latin 'genu' (knee) with 'flectere'. The family of words is enormous and remarkably consistent: wherever you see the syllable 'flect' or 'flex' in English, you are almost always looking at this ancient Latin verb.
There is something revealing about the metaphor built into 'inflexible'. When the Romans and later English speakers wanted to describe stubbornness, they reached not for a word about walls or stone, but for a word about bending. This tells us something about how they understood human nature: a good mind or character was supposed to have 'flexibilitas' — the ability to bend with circumstances, to adapt, to compromise where necessary. To be 'inflexibilis' was to be like a dry stick rather than a green branch — technically strong, perhaps, but likely to snap rather than yield. Stubbornness and rigidity were seen as a kind of brittleness.
Over the centuries, the word has kept both its physical and its metaphorical meanings with remarkable stability. In science and engineering, an inflexible material is one that does not flex under pressure. In everyday life, an inflexible person is one who will not bend their opinions, rules, or schedule. What is striking is how little the word has drifted from its Latin origin — unlike many words that travel across languages and centuries and arrive almost unrecognisable, 'inflexible' still means almost exactly what 'inflexibilis' meant in ancient Rome: something that refuses, or is unable, to bend.