Immemorial

Etymology

From Medieval Latin immemorialis, built from Latin in-, "not," and memorialis, "of memory," from memoria, "memory." The literal sense is "beyond memory." It survives most often in the legal and ceremonial phrase "time immemorial," which in English law once meant a time before formal records began, so remote that its origin cannot be traced.

adjective · /im-uh-MOR-ee-uhl/
So old that its beginning is beyond memory or record.
What it names
The word for antiquity without a known start — a custom from time immemorial — something so long-standing that no one can recall when it began.
Example
The villagers had gathered at the well on midsummer’s eve from time immemorial, though none could say who had started the ritual.
Related idea
Immemorial stresses that the origin is lost to memory; primordial stresses that a thing dates to the actual beginning of the world, whether or not anyone remembers it.