grapevine (n.)

also grape-vine, 1736, from grape + vine. Meaning "a rumor; a secret or unconventional method of spreading information" (1863) is from the use of grapevine telegraph as "secret source of information and rumor" in the American Civil War; in reference to Southerners under northern occupation but also in reference to black communities and runaway slaves.

The false reports touching rebel movements, which incessantly circulated in Nashville, brings us to the consideration of the "grapevine telegraph"—a peculiar institution of rebel generation, devised for the duplex purpose of "firing the Southern heart," and to annoy the "Yankees." It is worthy of attention, as one of the signs of the times, expressing the spirit of lying which war engenders. But it is no more than just to say that there is often so little difference between the "grapevine" and the associated press telegraph, that they might as well be identical. ["Rosecrans' Campaign with the Fourteenth Corps," Cincinnati, 1863]