An expression used in horse racing form analysis to indicate that previous good form is legitimate.
For example, if in Horse A's last start he beat Horse X in the Scenario Stakes, and Horse X has since won a different race, then Horse X has franked the Scenario Stakes form.
Therefore, Horse A has a good chance of winning his next start because his previous form has been franked, that is, it is a legitimate and trustworthy form reference.
"Can't believe the juicy price the bookies are offering for Humidor. Last start he finished half a length behind Winx, who franked that form by winning the Cox Plate by 5 lengths last week!"
