Unique weapons are the ones whose names are in purple and usually in their description have a story associated with them. Their names will also be more than just Strengthening Hammer or Fine Azurite Daggers. Examples are: Earth Gavel or Rimescythe. You can find or buy these weapons all over The Faelands and it isn't that hard to acquire them. You can even store a few in your chest in Canneroc or Adessa (should you make it that far before the achievement pops). Though the game says purple weapons are rare, however only 18 hours into the game I obtained the achievement. With a good amount of exploring this achievement will come with ease.
Edit by way of Cyber Grimm: as long as you have picked up the purple weapons and have had them in your inventory at some time this achievement will pop. So feel free to sell them if you have no need for them or salvage the parts of you are trying to build a better weapon.