Bothan stunner was the generic nickname for the Bothan stun stick, a type of melee weapon not unlike a taser. Basically, it was a short, unwieldy club intended to disable a victim for easy handling, pumping upwards of 582,797,754 volts of electricity into the hapless wienie.
These weapons were favored by Bothans, not surprisingly, and anyone else who wished to incapacitate some poor sap without drawing attention to themselves. A more powerful variant was known as the Chuka.
The "Bothan stunner" was also the finishing move of the galactic wrestler Stone Cold Steve Bothan.