On the grapevine means by word of mouth passed informally from person to person. I heard on the grapevine.
It always takes heard, and the source is never named because there is not one. News that travels on the grapevine is faster than any announcement and less reliable.
1phraseon the grapevineBy word of mouth passed informally from person to person.
“She heard on the grapevine that the office was closing.”
on the grapevine is a phrase.
Practised in Idioms