immemorial/ˌɪməˈmɔ:rɪəl; ˌɪmə`mɔrɪəl/ adj(fml or rhet 文或修辞)1 going back beyond the reach of human memory or written records （因年代久远）人类无法追忆的或文字记录所不及的.2 (idm 习语) from/since time immemorial => time.
牛津英汉双解词典