Disarmed is the state of being without your main hand weapon. This occurs when a mob or other player disarms you using abilities such as 
[Disarm], 
[Riposte], 
[Snatch], or 
[Dismantle]. You will be without your weapon for several seconds and unable to use any abilities that require a main hand weapon.
In Classic, being disarmed also temporarily makes you use the Unarmed weapon skill.
See also
For the achievements
For the daily fishing quest in Dalaran
[10-30 Daily] Disarmed!