hear something on/through the grapevine

If you hear some news or information on or through the grapevine, someone else tells it to you unofficially, often in conversation.

EXAMPLES:

"Who told you I was moving house?" "Oh, I just heard it on the grapevine."

Freddie was distressed when, through the grapevine, he heard of Liza's marriage.

I heard through the grapevine that she was pregnant, but I don't know anything more.

According to the high-school grapevine, Kelly wants me to ask her out on a date.