I would assume we refer to the Star Wars version of the disruptor rifle.
A disruptor rifle was a vicious and devastating weapon that disintegrated its target. It tore apart its victim at the molecular level, destroying their body tissues and bypassing most sorts of personal energy shields. A disruptor weapon shot a constant beam of energy that would travel in a straight direction until it hit solid matter.
Energy shields are made of plasma. Lightsaber blades are also made of plasma. Disruptor beams are made of highly concentrated plasma. It stands to reason that if a disruptor beam can pass through a plasma shield, then a lightsaber should be either similarly useless or ineffective at best.
This is demonstrated in the game Jedi Outcast. A DXR-6 disruptor rifle fires a charged beam. Unlike normal blaster bolts, a disruptor bolt from this rifle cannot be deflected by the lightsaber, only absorbed, albeit with difficulty. Quick shots cannot be easily anticipated and absorbed, and often not completely, therefore still inflicting some damage to a Jedi protected only by a lightsaber. At full charge, however, it can cause heavy damage even to a Jedi, and if not successfully blocked, it can still cause complete vapourisation altogether.