Check the "Race" tab when you're checking an item.If it is the "Giant's Sword", only a Mook can equip it. IIRC, the Mook can be Fighter,Lord,Valk,Samurai and Ranger to equip the Giant's Sword.
Similar to but not as restrictive as the Cane of Corpus.
Spence
As stated above, only a Faerie Ninja can use the Cane of Corpus. It is thought by most to be the best single handed, short range staff in the game. It's a cursed item though. When equiped, it forces you to use either magic or toss powders,potions and magic sticks for long/extended range attacks.
The rattkin boss, Don Barlone, has it on him. You have to either kill him for it or if your pick-pocket skill is high enough AND you don't have the latest patch(pick-pocket is nerfed) you may be able to steal it from him. It's a random drop but the chances of getting it is very high if you take him out.
I highly recommend you save before attempting to take him out.
He's tough if your guys are low levels and you may not get it the first time you kill him.
Spence