immemorial adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

late 14c., "fame, renown, reputation," also "a commemorative gesture, monument, or rite;" in general, "something by which the memory of a person, thing, or event is preserved," from Old French memorial "record, report," and directly from Late Latin memoriale "a memorial," noun use of neuter of Latin memorialis (adj.) "of or belonging to memory," from memoria "memory" (from PIE root *(s)mer- (1) "to remember").

Meaning "memorial act, commemorative gesture, monument, etc.," is from late 14c., as is the sense of "a written representation of facts made to a legislative or other body as the grounds for a petition." Related: Memorialist.

The old Man said, “I see around me here

Things which you cannot see: we die, my Friend,

Nor we alone, but that which each man loved

And prized in his peculiar nook of earth

Dies with him or is changed, and very soon

Even of the good is no memorial left.

[Wordsworth, "The Ruined Cottage"]

"not memorable," 1550s, from Latin immemorabilis "not worth mentioning; silent," from assimilated form of in- "not" (see in- (1)) + memorabilis (see memorable). In English it occasionally has been used to mean "old beyond memory," but that sense is best left to immemorial.