After Samuel Morse invented the telegraph in 1844, there was a lot of public fascination about the then-new form of communication in the years that followed.  People couldn’t help but compare telegraph communication with the one form of communication that was most prevalent at the time, word-of-mouth. People observed that the telegraph wires resembled the vines and tendrils of grape plants in terms of thickness, but unlike the grapevines that were messy and twisted, the telegraph lines were very straight. Building on these comparisons, people started to jokingly refer to word-of-mouth communication as “the grapevine telegraph.” So, instead of saying “I heard a rumor,” or “I heard it through gossip,” people would say “I heard it through the grapevine.” 

Even as other means of communication took over and people forgot about the telegraph, the term “grapevine” stuck around, and today we still use it to mean “gossip.”