Heard it through the grapevine
An indication that a piece of information was obtained via an informal contact.
The first practical public demonstration of the telegraph was given in 1844, when Samuel Morse sent a message from Washington to Baltimore. The invention was widely welcomed as a means of rapidly communicating news. It soon became clear though that close communities already had effective word-of-mouth communications. Soon after the telegraph was invented the term 'grapevine telegraph' was coined - first recorded in a US dictionary in 1852. This distinguished the new direct 'down-the-wire' telegraph from the earlier method, which was likened to the coiling tendrils of a vine. It's clear that the allusion was to interactions amongst people who could be expected to be found amongst grapevines, that is, the rural poor.
In 1876, The Reno Evening Gazette ran an article about a bumper corn and grape crop. They commented on the fact that the people who were then called Indians and Negroes seemed to be already aware of it (hardly a surprise you might think as it would have been they who had harvested the crops):
"It would seem that the Indians have some mysterious means of conveying the news, like the famous grapevine telegraph of the negroes in the [American Civil] war. The Pioneer Press and Tribune says that, while the first telegraphic news of Custer's death reached them at midnight, the Indians loafing about town were inquiring about it at noon."
The term 'bush telegraph' originated in Australia, probably influenced by 'grapevine telegraph'. That referred to the informal network that passed information about police movements to convicts who were hiding in the bush. It was recorded in 1878 by an Australian author called Morris:
"The police are baffled by the number and activity of the bush telegraphs."
In the UK it was the 'jungle telegraph' - referring to communications in outposts of the British Empire around the same period.
Of course 'heard it through the grapevine' is best known to us as the Motown song, recorded by Gladys Knight & the Pips in 1967 and by Marvin Gaye in 1968. It's salutary that, whilst the telegraph is long gone, the person-to-person communication that preceded it is still going strong.