Fear, Uncertainty, and Doubt. Contrary to FOMO, which pulls the interest of investors, FUD refers to panic selling in case of bad news, large price drops etc ...
Just like in case of FOMO, false FUD news are being spread too, but this time to manipulate prices to go down. Basically any unverified negative news that people spread over the net and other channels, is considered FUD news.
Youg markets such as Crypto, are extremely responsive to FUD news, legit or not, fresh or old ... FUD usually takes the prices down at least a bit.