"平水之交" 既英文係乜??

acquaintance
根據 Webster online dictionary:
Acquaintance: a person whom one knows but who is not a particularly close friend


2007-10-09 19:20:19 補充：
Nothing offensive. For discussion's sake, 香港人喜歡用Common friend來形容普通朋友，但老外所指的common friend，是指我和你都識的朋友，common 在這裡是共同咁解，而唔係普通。唔信可以打『common friend』 search下外國網頁。

Hope the below helps you:

平水之交 - acquaintant, common friend
Eg: He is just a mere acquaintant, we are not very closed to each other even we know each other for sometime.
OR
He is just a common friend as we do not have any close friendship.