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. Read more about it here . Today we use the phrase to mean we heard some information through an unofficial source.