Daily Chinese Idiom Guessing Game
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.