Time immemorial is a phrase meaning time extending beyond the reach of memory, record, or tradition, indefinitely ancient, "ancient beyond memory or record". The phrase is one of a few cases in the English language where the adjective is postpositive—some other phrases, such as the legal terms attorney general and court-martial, also follow that pattern, largely due to the influence of Norman French.
In law, it means that a property or benefit has been enjoyed for so long that its owner does not have to prove how they came to own it. In English law and its derivatives, time immemorial means the same as time out of mind, "a time before legal history and beyond legal memory." In 1275, by the first Statute of Westminster, the time of memory was limited to the reign of Richard I (Richard the Lionheart), beginning 6 July 1189, the date of the King's accession. Since that date, proof of unbroken possession or use of any right made it unnecessary to establish the original grant under certain circumstances. In 1832, time immemorial was re-defined as "Time whereof the Memory of Man runneth not to the contrary." 
- The dictionary definition of time immemorial at Wiktionary