Stands for fear, uncertainty, and doubt.
FUD is a disinformation strategy that is used as a weapon against competitors. This could also mean the spreading of false or harmful information to influence perception and appeal to their fear.
In hacking, FUD could also mean fully undetected. This either means (1) data that has been made to appear like random noise due to encryption, or (2) a piece of software that cannot be detected by AV tools and scanners.