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late 14c., "incapable of being bent, physically rigid," also figuratively, "unbending in temper or purpose," from Middle French inflexible and directly from Latin inflexibilis, from inflexus, past participle of inflectere (see inflect). In early 15c. an identical word had an opposite sense, "capable of being swayed or moved," from in- "in, on." Related: Inflexibly.