Immemorial, a.
Etymology: [Pref. im- not + memorial: cf. F. immémorial.]
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. 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.
Shades of meaning
Fernald has no entry of its own for immemorial; he lists it under old, primeval.
Where he writes about it:
Primeval (L. primum, first, and aevum, age), signifies strictly belonging to the first ages, earliest in time, but often only the earliest of which man knows or conceives, immemorial. — from primeval
Immemorial refers solely to time, independently of quality, denoting, in legal phrase, "that whereof the memory of man runneth not to the contrary;" as, an immemorial custom; an immemorial abuse. — from primeval