Emily Dickinson Lexicon

IM-ME-MO'RI-AL, a. [Fr. from L. in and memor, memoria.]

Beyond memory; an epithet given to time or duration, &c., whose beginning is not remembered, or can not be traced and ascertained; as, when it is said a man has possessed an estate in fee from time immemorial, or time out of mind. Such possession constitutes prescription, or prescriptive right. So we speak of immemorial use, custom or practice. In England, a thing is said to be immemorial, when it commenced before the reign of Edward II.