Fig.: to learn of something informally through gossip and rumors, usually by word of mouth.

I heard through the grapevine that our company will undergo a considerable downsizing of personnel. It might be just gossip, but I'd better start looking for a new job anyway

Ho sentito dire che la nostra ditta subirà una pesante ristrutturazione del personale. Magari sono solo voci, però è meglio che cominci a guardarmi attorno per cercare un altro lavoro

Divorce? I'm getting a divorce? You must have heard it through the grapevine because it's absolutely not true

Divorzio? Io starei divorziando? Sappi che sono solo pettegolezzi perché non è assolutamente vero

I heard through the grapevine that Ornella "had" to get married

Ho sentito dire che Ornella si sia "dovuta" sposare

The phrase, coined in the USA in the mid 1800s, provided an ironic comparison between the new marvel the telegraph, with its fast and straight way of communication, and the grapevine telegraph, which worked person to person, often distorting the facts or reporting plain rumors.
The term became widely known during the American Civil War period, when Major James Connolly mentioned the grapevine telegraph in his Diaries: "We get such "news" in the army by what we call "grape vine", that is, "grape vine telegraph." It is not at all reliable."