immemorial

US /ˌɪm.əˈmɔːr.i.əl/
UK /ˌɪm.əˈmɔːr.i.əl/
1.

xưa

existing or traditional for an extremely long time:

:
•
She said it was the immemorial custom of the residents to have a feast after the harvesting.
:
since time immemorial