Origin: 1350–1400; ME < L
inflexibilis rigid, unbending.
See in- 3 , flexible 
Related forms: in⋅flex⋅i⋅bil⋅i⋅ty, in⋅flex⋅i⋅ble⋅ness, noun
in⋅flex⋅i⋅bly, adverb
Synonyms:
1. unbendable, stiff. 2. rigorous, stern, unrelenting, unremitting, stubborn, obstinate, intractable, obdurate, unbending, adamant. Inflexible, relentless, implacable, inexorable imply having the quality of not being turned from a purpose. Inflexible means unbending, adhering undeviatingly to a set plan, purpose, or the like: inflexible in interpretation of rules; an inflexible will. Relentless suggests so pitiless and unremitting a pursuit of purpose as to convey a sense of inevitableness: as relentless as the passing of time. Implacable means incapable of being placated or appeased: implacable in wrath. Inexorable means unmoved by prayer or entreaty: inexorable in demanding payment. 3. undeviating.
Antonyms:
2. amenable.