An old acquaintance; someone you have known for a long time.
彼は大学時代の旧知で、久しぶりに連絡を取り合った。
He is an old acquaintance from university, and we got back in touch after a long time.
旧知の間柄だから、遠慮なく頼んでいいよ。
We're old acquaintances, so you can ask without holding back.
政治家同士が旧知だと報じられた。
It was reported that the politicians were old acquaintances.