Back in 1999, members of the football365 football forum (now mostly found at cupwinkcook.com) formed an e-mail group for some of the forummers to send each other the latest e-mail jokes, attachments, etc. in a convenient way.
One member of the group, Peacock (named so after the Leeds United nickname "The Peacocks"), had the rather annoying habit of posting the latest jokes, attachments, etc. after they had already been posted.
This led to the term "Peacocked".
*old joke arrives in Inbox*