This is an old achievement, no longer in the game (but if you'd earned it before, it will show up under Feats of Strength).
You were able to obtain it by defeating Cydaea without wearing any armor on Hell difficulty or higher. Even if you were playing with friends, you'd still get the achievement so long as you were naked while fighting - and some speculated that you needed to get the killing blow, but I don't think that was the case.
