/inisipe1/ > /inisiɸe/ > /inisiwe/ > /inisie/. Literally "that which has passed". From the verb 往ぬ (inu, “pass”) in adverbial form ini, the past marker き (ki) in attributive form si, and へ (pe, “time, direction”).
いにしえ • (inishie) ←いにしへ (inisife)?