A tall tale.
A rhyming slang phrase dating back to Mediaeval times when Sir Guy of Gisburne spread unfounded rumours about Robin of Loxley (aka Robin Hood).
The phrase "You're telling a Gisburne" has been adulterated over the years, and its rhyming equivalent (a "Lisbon") has gained popularity wihtin cyber-space message boards.
"I don't believe you!! You're telling a lisbon!"