Ginn-su Sword is a rare one handed, dual wield capable sword. It has bonuses to agility and strength, making it ideal for Melee DPS classes, and, as such it is popular for lvl 39 twinks. Its in-game model looks much like a mini katana.
It is particularly saught after because it is one of the rare swords that, when sheathed, is wielded on the character's back; much like the Felsteel Longblade is. This cosmetic advantage tends to make it even more expensive beyond it's twinking capabilities.
Is a random drop off the mobs in Uldaman.
The name is probably a reference to the Ginsu knives.