The Collaborative International Dictionary of English v.0.48:

Immutable \Im*mu"ta*ble\, a. [L. immutabilis; pref. im- not +
   mutabilis mutable. See Mutable.]
   Not mutable; not capable or susceptible of change;
   unchangeable; unalterable.
   [1913 Webster]

         That by two immutable things, in which it was
         impossible for God to lie, we might have a strong
         consolation.                             --Heb. vi. 18.
   [1913 Webster]

         Immutable, immortal, infinite,
         Eternal King.                            --Milton.
   -- Im*mu"ta*ble*ness, n. -- Im*mu"ta*bly, adv.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

256 Moby Thesaurus words for "immutable":
   abiding, accordant, adamant, adamantine, age-long, aged, alike,
   all-knowing, all-powerful, all-seeing, all-wise, almighty, ancient,
   antique, balanced, boundless, cast-iron, changeless, chronic,
   consistent, consonant, constant, continuing, continuous,
   correspondent, creating, creative, diligent, diuturnal, dogged,
   dour, durable, enduring, equable, equal, eternal,
   eternally the same, even, evergreen, everlasting, faithful, firm,
   fixed, flat, flinty, frozen, glorious, good, grim, hallowed, hard,
   hard-core, hardy, highest, holy, homogeneous, immobile, immortal,
   immovable, implacable, impliable, inalterable, incommutable,
   inconvertible, indefatigable, indefeasible, indomitable, inductile,
   industrious, inelastic, inert, inexorable, inextensible,
   inextensile, inextensional, infinite, inflexible, insistent,
   insusceptible of change, intact, intractable, intractile,
   intransient, intransigent, intransmutable, invariable, inveterate,
   invincible, inviolate, iron, irreconcilable, irresilient,
   irretrievable, irreversible, irrevocable, just, lasting, level,
   limitless, long-lasting, long-lived, long-standing, long-term,
   longeval, longevous, loving, loyal, luminous, macrobiotic,
   majestic, making, measured, mechanical, merciful, methodic,
   monolithic, never-tiring, noble, nonelastic, nonreturnable,
   nonreversible, nonstretchable, numinous, obstinate, of a piece,
   of long duration, of long standing, omnipotent, omnipresent,
   omniscient, one, ordered, orderly, patient, patient as Job,
   perdurable, perduring, perennial, permanent, perpetual,
   perseverant, persevering, persistent, persisting, pertinacious,
   plodding, plugging, preoccupied, quiescent, radiant, rapt, regular,
   relentless, remaining, resolute, reverseless, rigid, rigorous,
   robotlike, rock-ribbed, sacred, sedulous, sempervirent, shaping,
   single-minded, sleepless, slogging, smooth, solid, sovereign,
   stable, static, stationary, staying, steadfast, steady, steely,
   stern, stiff, stubborn, supreme, sustained, systematic, tenacious,
   timeless, tireless, torpid, tough, ubiquitous, unabating,
   unaffected, unalterable, unalterative, unaltered, unbending,
   unbounded, unbroken, unchangeable, unchanged, unchanging,
   unchecked, uncompromising, unconquerable, undaunted, undefined,
   undeflectable, undestroyed, undeviating, undifferentiated,
   undiscouraged, undiversified, undrooping, unextendible,
   unextensible, unfading, unfailing, unfaltering, unflagging,
   unflexible, unflinching, ungiving, uniform, unintermitting,
   uninterrupted, unlimber, unlimited, unmalleable, unmodifiable,
   unmovable, unmoved, unnodding, unpliable, unpliant, unrelaxing,
   unrelenting, unremitting, unrestorable, unreturnable, unruffled,
   unshifting, unsleeping, unsusceptible, unswerving, untiring,
   untractable, unvariable, unvaried, unvarying, unwavering,
   unwearied, unwearying, unwinking, unyielding, utterly attentive,
   vital, weariless