The Collaborative International Dictionary of English v.0.48:

Unyielding \Unyielding\
   See yielding.
   ----- and the like.
   [1913 Webster]

   Note: The above classes of words are unlimited in extent, and
         such compounds may be formed by any writer or speaker
         at will from almost all the adjectives or participles
         in the language, excepting those which have a
         recognized and usual negative correspondent with the
         prefix -in. No attempt will be made, therefore, to
         define them all in this Dictionary; many will be
         omitted from its Vocabulary which are negations of the
         simple word, and are readily explained by prefixing a
         not to the latter. Derivatives of these words in -ly
         and -ness will also, for the most part, be omitted for
         the same or similar reasons.
         [1913 Webster] There will be inserted as separate
         articles with definitions, the following:
         [1913 Webster] 1. Those which have acquired an opposed
         or contrary, instead of a merely negative, meaning; as,
         unfriendly, ungraceful, unpalatable, unquiet, and the
         like; or else an intensive sense more than a prefixed
         not would express; as, unending, unparalleled,
         undisciplined, undoubted, unsafe, and the like.
         [1913 Webster] 2. Those which have the value of
         independent words, inasmuch as the simple words are
         either not used at all, or are rarely, or at least much
         less frequently, used; as, unavoidable, unconscionable,
         undeniable, unspeakable, unprecedented, unruly, and the
         like; or inasmuch as they are used in a different sense
         from the usual meaning of the primitive, or especially
         in one of the significations of the latter; as,
         unaccountable, unalloyed, unbelieving, unpretending,
         unreserved, and the like; or inasmuch as they are so
         frequently and familiarly used that they are hardly
         felt to be of negative origin; as, uncertain, uneven,
         and the like.
         [1913 Webster] 3. Those which are anomalous,
         provincial, or, for some other reason, not desirable to
         be used, and are so indicated; as, unpure for impure,
         unsatisfaction for dissatisfaction, unexpressible for
         inexpressible, and the like.
         [1913 Webster]
   II . Un- is prefixed to nouns to express the absence of, or
      the contrary of, that which the noun signifies; as,
      unbelief, unfaith, unhealth, unrest, untruth, and the
      like.
      [1913 Webster]

   Note: Compounds of this last class are given in full in their
         proper order in the Vocabulary.
         [1913 Webster]

WordNet (r) 3.0 (2006):

unyielding
    adj 1: stubbornly unyielding; "dogged persistence"; "dour
           determination"; "the most vocal and pertinacious of all
           the critics"; "a mind not gifted to discover truth but
           tenacious to hold it"- T.S.Eliot; "men tenacious of
           opinion" [syn: dogged, dour, persistent,
           pertinacious, tenacious, unyielding]
    2: resistant to physical force or pressure; "an unyielding head
       support"

Moby Thesaurus II by Grady Ward, 1.0:

225 Moby Thesaurus words for "unyielding":
   adamant, adamantine, at a standstill, bowelless, bulky, bullheaded,
   cast-iron, certain, closed to, complaining, constant, cruel, dense,
   disputatious, disputing, dissentient, dissenting, dog-eat-dog,
   dour, durable, enduring, fated, fateful, firm, fixed, flinty,
   fractious, frozen, fundamentalist, grim, hard, hard-core,
   hard-line, harsh, headstrong, heartless, heavy, hidebound,
   immobile, immotile, immotive, immovable, immutable, impenetrable,
   impersuadable, impersuasible, impervious, implacable, impliable,
   impregnable, inclement, incontestable, indefeasible, indomitable,
   inductile, inelastic, ineluctable, inescapable, inevasible,
   inevitable, inexorable, inexpugnable, inextensible, inextensile,
   inextensional, inflexible, insuperable, intractable, intractile,
   intransigent, invincible, inviolable, invulnerable, iron,
   ironbound, ironclad, ironfisted, ironhanded, irreconcilable,
   irremovable, irresilient, irresistible, irrevocable, lasting,
   loyal, massive, merciless, mortal, mulish, muscle-bound, necessary,
   noncooperative, nonelastic, nonstretchable, obdurate, objecting,
   obstinate, obstructive, on the barricades, orthodox, overpowering,
   overwhelming, pat, pertinacious, pigheaded, pitiless, procrustean,
   proof against, protesting, purist, puristic, puritan, puritanic,
   rebellious, recalcitrant, refractory, relentless, reluctant,
   remorseless, renitent, repellent, resistant, resisting, resistive,
   resistless, retardant, retardative, rigid, rigorist, rigoristic,
   rigorous, rock-ribbed, rockbound, rugged, ruthless, self-willed,
   set, settled, single-minded, solid, sot, sound, stable, standpat,
   stationary, staunch, steadfast, steady, steely, stern, stiff,
   stout, straightlaced, straitlaced, strong, stubborn, sturdy, sure,
   sure as death, sure as fate, tough, true, unaffected, unalterable,
   unamenable, unappeasable, unassailable, unavoidable, unbeatable,
   unbending, unchangeable, uncompassionate, uncompassioned,
   uncompliant, uncompromising, unconquerable, uncontrollable,
   uncooperative, undeflectable, unextendible, unextensible,
   unfeeling, unflappable, unflexible, unflinching, unforgiving,
   ungiving, unimpressionable, uninfluenceable, unlimber, unmalleable,
   unmerciful, unmovable, unmoved, unmoving, unpersuadable, unpitiful,
   unpitying, unpliable, unpliant, unpreventable, unreceptive,
   unrelenting, unremorseful, unresponsive, unshaken, unstoppable,
   unsubduable, unsubmissive, unsuggestible, unsurmountable,
   unsusceptible, unswayable, unswerving, unsympathetic,
   unsympathizing, untractable, up in arms, well-built,
   well-constructed, well-founded, well-grounded, well-made,
   without mercy, withstanding