/ ˌɪməˈmɔːriəl; NAmE / adjective
( formal or literary ) that has existed for longer than people can remember :
an immemorial tradition
My family has lived in this area from time immemorial (= for hundreds of years) .
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WORD ORIGIN
early 17th cent.: from medieval Latin immemorialis , from in- not + memorialis relating to the memory.