That doesn't necessarily mean costumes...
Actually, judging by the "Undertaker's Staff" item, i'd say the text belongs to it; I have no idea how to use it though.
Anyway, for that knife, i'm gonna take a guess and say it means there's a chance of stealing an item when attacking.. think like the "Thief's Knife" from old Final Fantasy games. That's my guess, at least.
*edit*
After some testing, that's exactly what it does. The only difference with this and the FF one is that it probably checks stealing skill. Also, you have to not be carrying a shield for it to work.