DictionaryRoot: memorialis
Latin
memorialis
of or belonging to memory
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adjective
immemorial
im-uh-MOR-ee-ul/ˌɪməˈmɔːriəl/
Extending so far into the past that no one remembers its beginning.
Etymology
The negative prefix places something outside the reach of memory. Here that means its beginnings are too ancient to be remembered, rather than that it is unworthy of remembrance.