The “grapevine” (or “grape-vine”) is an often-secret information network. The “grapevine” information was so unreliable that “grapevine” became synonymous with “rumor.” The name comes from “grape-vine telegraph”—cited in print since 1852—because the telegraph wires resembled tangled grapevines. The “grape-vine” became a popular information network term during the underground railroad of the 1850s and the Civil War in the 1860s.
 
The 1960s song, “I Heard It Through the Grapevine,” became a hit recording for Marvin Gaye and further popularized the “grapevine” term. The news coming through the song’s “grapevine” lyrics is about an unfaithful companion.
 
A “political grapevine”—cited in print since 1910—means a source of political rumors. The Fox News television show Special Report features a daily “Political Grapevine” segment.
 
     
Wikipedia: Grapevine (gossip)
To hear something through the grapevine is to learn of something informally and unofficially by means of gossip and rumor.
 
The usual implication is that the information was passed person to person by word of mouth, perhaps in a confidential manner among friends or colleagues. It can also imply an overheard conversation or anonymous sources of information. For instance “I heard through the grapevine that Brad was getting fired.”
 
Etymology
In the autobiography of Booker T. Washington, Up From Slavery, Washington says that slaves in the South kept up-to-date on current events by “what was termed the ‘grape-vine’ telegraph.” He said,
 
“Often the slaves got knowledge of the results of great battles before the white people received it. This news was usually gotten from the coloured man who was sent to the post office for the mail… The man who was sent to the post office would linger about the place long enough to get the drift of the conversation from the group of white people who naturally congregated there, after receiving their mail, to discuss the latest news. The mail carrier on his way back to our master’s house would as naturally retail the news that he had secured among the slaves, and in this way they often heard of important events before the white people at the ‘big house,’ as the master’s house was called.”
 
According to Jitendra Mishra:
 
“The term grapevine can be traced to Civil War days when vinelike telegraph wires were strung from tree to tree across battlefields and used by Army Intelligence. The messages that came over these lines were often so confusing or inaccurate that soon any rumor was said to come from the grapevine. Usually, grapevines flow around water coolers, down hallways, through lunch rooms, and wherever people get together in groups. The lines of communication seem to be haphazard and easily disrupted as the telegraph wires were, however, they transmit information rapidly and in many cases faster and with a stronger impact than the formal system allows.”
 
The term gained a boost in popularity through its use in the Motown song “I Heard It Through the Grapevine”, a major hit single for both Marvin Gaye and Gladys Knight & the Pips in the late 1960s.
 
Grapevine/Informal Communication
The term grapevine communication is often used interchangeably with the term informal communication. The term originated in the 1860s during the American Civil War. It was used as a term that described the telegraph lines that were strung through the trees in a manner that resembled grapevines. It also came to mean informal communication that was not very effective because the telegraph system was not a reliable source of communication at the time. Almost a century later, it was discovered that the path of grapevine communication does resemble a cluster of grapes.
 
Wikipedia: I Heard It Through the Grapevine
“I Heard It Through the Grapevine” a landmark song in the history of Motown Records. Written by Norman Whitfield and Barrett Strong in 1966, the single was first recorded by Smokey Robinson & the Miracles. Released on September 25, 1967 as Soul 35039 by Gladys Knight & the Pips, who recorded the third version of the song, it has since become a signature song, however, for singer Marvin Gaye, who recorded his version of the song prior to the Pips’ but released it after theirs on October 30, 1968 as Tamla 54176. Creedence Clearwater Revival released their popular version of the song in 1970. It was referenced in Talkin’ ‘Bout Your Generation.
 
Gaye’s version has since become a landmark in pop music. In 2004, it ranked #80 on Rolling Stone‘s list of The 500 Greatest Songs of All Time. On the commemorative 50th Anniversary of the Billboard Hot 100 issue of Billboard magazine in June 2008, Gaye’s version was ranked as the 65th biggest song on the chart. It was also inducted to the Grammy Hall of Fame for “historical, artistic and significant” value.
 
(Historical Dictionary of American Slang)
grapevine n. [short for GRAPEVINE TELEGRAPH]
1.a. any informal or unofficial method of relaying important or interesting information, esp. by word of mouth; the means by which gossip or rumor travels. Also quasi-adj., (of a report) learned confidentially. Now S.E.
1862 in Conolly

25: We get such “news” in the army by what we call “grape-vine,” that is “grape vine telegraph.” It is not all reliable.
1863 in Sperber 7 Trittschuh Amer. Pol. Terms 179: A despatch came from Richmond “via grapevine.”
1864 in War of the Rebellion (ser.II) VII: They pretended to have “grapevine” intelligence that [the raiders] were up as far north as the Missouri River.
1.b. gossip, rumor, or news; (also) a rumor. Also quasi-adj., being the nature of an untrustworthy rumor.
1863 in Connolly Army of Cumberland 48: it is “grape vine” that Grant’s and Burnside’s armies will unite with us within the next month.
1863 in F. Moore Rebel Rec. VI P33: It is just another grapevine.
(...)
grapevine telegraph n.
GRAPEVINE
1852 in Sperber & Trittschuh Amer. Pol. Dict. 178: By the Grape Vine Telegraph Line…we have received the following.
 
(Oxford English Dictionary)
grape-vine
Orig., a canard: current during the American civil war, and shortened from ‘a despatch by grape-vine telegraph’ (Funk’s Stand. Dict.). Now in general use to indicate the route by which a rumour or a piece of information (often of a secret or private nature) is passed.
a1867 B. F. WILLSON Old Sergeant vii. (Funk) Just another foolish grape-vine.
1891 Century Mag. Mar. 713/2 The ‘grape-vine’ spoke to us of little else.
1934 J. T. FARRELL Studs Lonigan (1936) II. xv. 337 Down there at that express company they find out about everything a guy does. They got the best grapevine in the world.
1948 Daily Tel. 3 Sept. 4/5 The guerrillas know the jungle, and they have an almost incredible ‘grapevine’ which gets information from one State to another with uncanny speed.
1955 Times 11 June 9/6 Of the younger men, the Moscow grape-vine reported that Mr. Shepilov, editor of Pravda, was coming forward to strengthen the party theoreticians.
     
22 June 1852, Trenton (NJ) State Gazette, “Interesting Political Correspondence,” pg. 1:
The Pittsburgh Dispatch makes itself responsible for the following jeu d’ esprit:
 
By the Grape Vine Telegraph line, in connection with Virginia Fence and Mason & Dixon’s Line, we have received the following interesting correspondence—far “ahead of the foremost,” which we hasten to lay before our readers:...
 
31 December 1862, Daily Delta (New Orleans, LA), “News from Southern Sources,” pg. 2:
The National Advocate of this city, a journal in the interest of the rebels, issued an extra yesterday afternoon, containing the essence of certain rebel papers, received by the editor through the “grape vine telegraph.”
 
Google Books
Loyalty on the Frontier, or, Sketches of Union men of the South-west with incidents and adventures in rebellion on the border
By Albert Webb Bishop
St. Louis, MO: R.P. Studley
1863
Pg. 210:
With rebels in our midst, it was to be supposed that the “grape-vine” telegraph was kept in active operation, and the expectation of attack was constant.
     
27 February 1863, Macon (GA) Telegraph, pg. 1:
Were we to listen to all the items of news which old Dame Rumor sends by the grapevine telegraph, we might entertain our readers with gossip, but we have already promised nothing but facts, and will therefore not publish the grapevine, for fear that some knot or crook in it may prevent the dispatch from being intelligible, and not to be relied upon.
 
11 March 1863, Daily Milwaukee News (Milwaukee, WI), pg. 1, col. 1:
It can hardly be expected under the circumstances that the state has been redeemed, but the grape vine telegraph we shall by degrees learn of a large democratic gains.
 
Google Books
Across the Continent:
A summer’s journey to the Rocky mountains, the Mormons, and the Pacific states
By Samuel Bowles
Springfield, MA: S. Bowles & Co.; New York, NY: Hurd & Houghton
1865
Pg. 62:
How it is done I do not understand — there must be a subtle telegraph by crinoline wires; as the southern negroes have what they call a grape-vine telegraph.
   
Google Books
A dictionary of slang, jargon & cant embracing English, American, and Anglo-Indian slang, pidgin English, tinker’s jargon and other irregular phraseology
By Albert Marie Victor Barrère and Charles Godfrey Leland
London: Ballantyne Press
1889-90
Pg. 425:
Grape-vine telegraph (AMerican). During the war exciting accounts of battles not fought and of victories not won were said to have been conveyed by grape-vine (or clothes-line) telegraph (New York Slang Dictionary), but the term was in earlier use, meaning news conveyed in a mysterious manner.
 
Chronicling America
24 August 1910, San Francisco (CA) Call, “Machine to Drop Independent Move” by George A. Van Smith, pg. 5, col. 2:
According to the political grapevine messages, the machine will not assume responsibility for a scheme that could have no other apparent purpose than the defeat of Johnson.
 
30 June 1916, Laurel (MS) Daily Leader, “Assassination of Progressive Party, Age of 4,” pg. 4, col. 1:
After the election the word began coming to the big insurgents by the political grapevine telegraph lines that Roosevelt was getting more and more insurgent in his views, and was beginning to have his doubts about Taft.
   
27 December 1921, Grand Forks (ND) Herald, pg. 4:
On the Political Grape Vine
 
Google News Archive
27 December 1934, St. Petersburg (FL) Evening Independent, “Delay in County Clerk Decision Surprises Many,” pg. 1, col. 8:
“Grapevine” Channels Busy
Gov. Sholtz had told O’Quinn at Tallahassee yesterday that Booth was the man who would take the post, said these political dopesters, who claimed to have received their information through the political “grapevine” channels from tho state capitol.
 
FOXNews.com - Special Report with Bret Baier
Political Grapevine: 6/8 (2010—ed.)