noun, adverb
ancient times; prehistoric times; the distant past; a very long time ago
Refers to a time far in the past, often with a sense of great antiquity. Can be used as a noun or adverbially.
大昔、この辺りは海だったらしい。
In ancient times, this area was apparently under the sea.
それは大昔の話
That's a story from the distant past.