through the grapevine 从非正式途径传来的；谣言；小道消息

【例句】
I heard through the grapevine that the minister is going to resign.
我听到小道消息，首相将辞职。

Was not even aware of their engagement until she heard it through the grapevine.
她是通过小道消息才得知他们已经订婚的。