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old friend; old acquaintance
旧知の仲なので遠慮なく頼み事ができる。
Since we're old friends, I can ask favors without hesitation.
海外で偶然旧知に出会って驚いた。
I was surprised to run into an old acquaintance abroad by chance.
旧知を頼って新しい仕事を紹介してもらった。
I relied on an old friend and got introduced to a new job.