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]

WordNet (r) 3.0 (2006):

immutable
    adj 1: not subject or susceptible to change or variation in form
           or quality or nature; "the view of that time was that all
           species were immutable, created by God" [syn:
           immutable, changeless] [ant: changeable, mutable]

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



Bouvier's Law Dictionary, Revised 6th Ed (1856):

IMMUTABLE. What cannot be removed, what is unchangeable. The laws of God
being perfect, are immutable, but no human law can be so considered.