"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.
immeasurable
immediacy
immediate
immediately
immediatism
immemorable
immemorial
immense
immensely
immensity
immensurable