To have never known each other before; to be complete strangers.
素平素，向来。向来不认识。
元·尚仲贤《柳毅传书》第四折我与你素不相识，一旦为你寄书，因而戏言，岂意遂为眷属。”
Even though they were total strangers, they helped each other like old friends.
Learn the next connected idiom: 素不相识 (sù bù xiāng shí) → 识文断字 (shí wén duàn zì)
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