Archaic noun meaning a very long span of time, often used in classical poetry and historical texts.
幾千代も昔の話だ。
It is a story from many ages ago.
Compound of 幾 (iku, 'how many') + 千 (chi, 'thousand') + 代 (yo, 'generation, age'). The exact historical derivation is uncertain, but the word is attested in classical Japanese.