A false promise, especially one which is on conditional terms that are difficult or impossible to check (at least, for the recipient of the promise), so that there's some measure of plausible deniability as to whether the promise was kept.
"They told us that we'd all get our bonuses if performance standards across the whole company rise by 15% this year. What a purple promise!"
"He's made us another purple promise in the election campaign, apparently he's going to 'help protect the environment' -- whatever that means."
The term originated as the name of an internal marketing strategy at FedEx wherein profit sharing bonuses are only given out if performance numbers meet certain goals throughout the whole company -- something which few employees would have the ability to actually check. Within the company, the plan has also been jokingly referred to as "The Purple Carrot (On A Stick)".