The grapevine is an informal and unofficial source of new information, gossip or rumours.
I heard it through the grapevine means you have heard information or rumours via a network of other people.
This expression is sometimes also I heard it on the grapevine.
Examples of use:
1. I heard it through the grapevine that Maria and Pedro are getting married.
2. How do you know that some people at work are going to lose their jobs?
I heard it through the grapevine.
3. We heard through the grapevine that you are selling your house and moving to Europe.
I heard it through the grapevine is the title of a famous song by Marvin Gaye. You can listen to the song, and read the lyrics, on this video:
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