‘Through the grapevine' is more commonly heard than 'On the grapevine’ in both social and professional contexts, and means that one hears something unofficially rather than through an official announcement. This is a term generally used during gossiping conversations, when someone is spreading rumours. Gladys Knight and The Pips released the single ‘I Heard it Through The Grapevine’ in 1967, but it was made into a worldwide hit when American singer/songwriter Marvin Gaye released the single in December 1968. His version went to number 1 in the American charts on release and was Gaye’s first number hit in his music career.
I heard it through the grapevine that our neighbour is pregnant again. I don’t know how she’ll cope, as she already has three children.
Our company is being sold later this year and we’re all going to lose our jobs. I heard it through the grapevine, so don’t tell anybody yet.