The Collaborative International Dictionary of English v.0.48:

Immemorial \Im`me*mo"ri*al\, a. [Pref. im- not + memorial: cf.
   F. imm['e]morial.]
   Extending beyond the reach of memory, record, or tradition;
   indefinitely ancient; as, existing from time immemorial.
   "Immemorial elms." --Tennyson. "Immemorial usage or custom."
   --Sir M. Hale.
   [1913 Webster]

   Time immemorial (Eng. Law.), a time antedating (legal)
      history, and beyond "legal memory" so called; formerly an
      indefinite time, but in 1276 this time was fixed by
      statute as the begining of the reign of Richard I. (1189).
      Proof of unbroken possession or use of any right since
      that date made it unnecessary to establish the original
      grant. In 1832 the plan of dating legal memory from a
      fixed time was abandoned and the principle substituted
      that rights which had been enjoyed for full twenty years
      (or as against the crown thirty years) should not be
      liable to impeachment merely by proving that they had not
      been enjoyed before.
      [1913 Webster]

WordNet (r) 3.0 (2006):

immemorial
    adj 1: long past; beyond the limits of memory or tradition or
           recorded history; "time immemorial"

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "immemorial":
   acknowledged, admitted, age-old, ageless, ancient, antique, auld,
   ceaseless, coeternal, constant, continual, continuous,
   conventional, customary, dateless, early, elderly, endless,
   erstwhile, established, eternal, eterne, ever-being, ever-durable,
   ever-during, everlasting, everliving, fixed, folk, fore, former,
   hallowed, handed down, heroic, hoary, incessant, indestructible,
   infinite, interminable, inveterate, late, legendary,
   long-established, long-standing, mythological, never-ceasing,
   never-ending, nonstop, nonterminating, nonterminous,
   of long standing, of old, of the folk, of yore, olamic, old,
   old as Methuselah, old as history, old as time, old-time, olden,
   once, onetime, oral, past, perdurable, permanent, perpetual,
   prehistoric, prescriptive, previous, primeval, primitive, prior,
   quondam, received, recent, recognized, rooted, sempiternal,
   sometime, steady, then, time-honored, timeless, traditional,
   tried and true, true-blue, unceasing, understood, unending,
   unintermitting, uninterrupted, unremitting, unwritten, venerable,
   without end, worshipful



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

IMMEMORIAL. That which commences beyond the time of memory. Vide Memory,
time of.