I always understood it to mean that the person would spread the news everywhere. It's best to keep things to yourself.
Today the jam crock is different but the message is the same. Social media today means that any thought can be shared instantly, often without sober second thought before touching the SEND or RETURN key.
Today we put information on the twenty-first century jam crock ourselves, or others, well intentioned or not, do it for us or to us. It seems as if there is little control of what makes it to the jam crock today but that was always the case. Today however, the speed with which items are shared is so much faster and the media used to transmit the items are so much more efficient and diverse, video, audio, stills, text, all via the internet.
"Today the jam crock is emblazoned with gossip," says the woman who writes a blog. What would Nan think?