Time out of mind; sometime so long ago that no-one can remember it. Usually used informally.
At one time (in 1275, to be exact), English law officially set time immemorial as 1189 or earlier, this date being the beginning of the reign of Richard I. Proof of unbroken possession or use of any right since that date made it unnecessary to establish the original grant.
In 1832 the idea of setting an exact date for time immemorial was abandoned. Instead, possession for 20 years (or 30, when claims were being made against the crown's holdings), were sufficient to grant a legal right to ownership. I believe that it is now set at 12 years, at least for ownership of land. This applies only in the UK; laws vary from state to state in the US.
In common law, one usually tries to prove that the possession, custom, etc., has lasted for as long as living memory can recall. The oldest living inhabitant of an area may be called on as a witness.
See adverse possession, Squatting: Law.