The Collaborative International Dictionary of English v.0.48:

Inflexible \In*flex"i*ble\, a. [L. inflexiblis: cf. F.
   inflexible. See In- not, and Flexible.]
   1. Not capable of being bent; stiff; rigid; firm; unyielding.
      [1913 Webster]

   2. Firm in will or purpose; not to be turned, changed, or
      altered; resolute; determined; unyieding; inexorable;
      stubborn.
      [1913 Webster]

            "Inflexibleas steel."                 --Miltom.
      [1913 Webster]

            A man of upright and inflexible temper . . . can
            overcome all private fear.            --Addison.
      [1913 Webster]

   3. Incapable of change; unalterable; immutable.
      [1913 Webster]

            The nature of things is inflexible.   --I. Watts.

   Syn: -- Unbending; unyielding; rigid; inexorable;
        pertinacious; obstinate; stubborn; unrelenting.
        [1913 Webster]

WordNet (r) 3.0 (2006):

inflexible
    adj 1: incapable of change; "a man of inflexible purpose" [ant:
           flexible]
    2: not making concessions; "took an uncompromising stance in the
       peace talks"; "uncompromising honesty" [syn:
       uncompromising, sturdy, inflexible] [ant:
       compromising, conciliatory, flexible]
    3: resistant to being bent; "an inflexible iron bar"; "an
       inflexible knife blade"; [ant: flexible, flexile]
    4: incapable of adapting or changing to meet circumstances; "a
       rigid disciplinarian"; "an inflexible law"; "an unbending
       will to dominate" [syn: inflexible, rigid, unbending]

Moby Thesaurus II by Grady Ward, 1.0:

160 Moby Thesaurus words for "inflexible":
   adamant, adamantine, at a standstill, cast-iron, certain,
   changeless, closed to, constant, determinate, determined, dogged,
   dour, dyed, established, fated, fateful, firm, fixed, flinty,
   frozen, fundamentalist, grim, hard, hard and fast, hard-core,
   hard-line, headstrong, hidebound, immobile, immotile, immotive,
   immovable, immutable, impersuadable, impersuasible, impervious,
   implacable, impliable, inalterable, indefeasible, indomitable,
   inductile, inelastic, ineluctable, inescapable, inevasible,
   inevitable, inexorable, inextensible, inextensile, inextensional,
   intractable, intractile, intransigent, invariable, invincible,
   iron, ironbound, ironclad, ironhanded, irreconcilable, irremovable,
   irresilient, irresistible, irrevocable, loyal, mulish,
   muscle-bound, necessary, nonelastic, nonstretchable, obdurate,
   obstinate, orthodox, pat, pigheaded, procrustean, purist, puristic,
   puritan, puritanic, refractory, relentless, resistless, resolute,
   resolved, rigid, rigorist, rigoristic, rigorous, rock-ribbed,
   rockbound, set, settled, severe, single-minded, sot, standpat,
   stationary, staunch, steadfast, steady, steely, stern, stiff,
   stony, straightlaced, straitlaced, strict, stubborn, sure,
   sure as death, sure as fate, true, unaccommodating, unadaptable,
   unaffected, unalterable, unamenable, unavoidable, unbending,
   unchangeable, unchanging, uncompliant, uncompromising,
   unconquerable, uncontrollable, undeflectable, unextendible,
   unextensible, unflappable, unflexible, ungiving, unimpressionable,
   uninfluenceable, unlimber, unmalleable, unmodifiable, unmovable,
   unmoved, unmoving, unpersuadable, unpliable, unpliant,
   unpreventable, unreceptive, unrelenting, unresponsive, unshakable,
   unshakeable, unshaken, unstoppable, unsuggestible, unsusceptible,
   unswayable, unswerving, untractable, unvaried, unvarying,
   unyielding