"Thief." is a red text modifier on weapons in Borderlands 2
Found on the following red text weapon: Incendiary Greed, (or any other version of the Greed Pistol) which drops from Roscoe in the Captain Scarlett and Her Pirate's Booty DLC
Grants Weapon Effects[edit]
You can aim down the sights without suffering a speed penalty
Greed Pistols are always fire elemental