To hear something through the grapevine is to learn of something informally and unofficially by means of gossip or rumor.
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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 his autobiography Up From Slavery, Booker T. Washington says that slaves in the South kept up-to-date on current events by "what was termed the 'grape-vine' telegraph."
However, the New York Public Library contends that the phrase derives from the infamous Grapevine Tavern in New York City's Greenwich Village. During the Civil War it "...was a popular hangout of Union officers and Confederate spies... It was the ideal place to get news and information, or in the case of spies and politicians, the ideal place to spread rumors and gossip, leading to the popular phrase 'heard it through the grapevine'."
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
Grapevine communication existed from the American Civil War to the First World War. It was coined this because of its nature of networking and reaching several at once; it causes the transformation of information between one individual and another.
Features
Flexibility: There is no formal control over grapevine, so it is more flexible than other forms of communication.
Rapid communication: It is faster than any form of communication.
No record: There is no evidence which can be documented for future reference
Distortion: The message which is passed gets distorted when it passes from one person to another.
Spontaneity: Grapevine communication is spontaneous as it is passed automatically from the top level of the organization to the bottom level without any difficulty in delivering the message. It is used by management to spread information that either cannot be shared officially or in an attempt to test the waters.
Cheap: It does not require any funds to take place.