Take the Discarded Knife to Quintis Jonespyre in Feathermoon Stronghold.
The Stronghold's resident druid, Quintis Jonespyre, has some... unusual skills. One of them involves psychometry - the ability to reveal an item's history by touch. Don't ask me how he does it; I chalk it up to magic I do not and choose not to understand. I do know, however, two things: one, I've seen it work and two, the bum owes me.
Take the knife up to Quint in his little tree-house here, and tell him for me that his Kalimdor Hold 'Em debt will be paid if he works his mojo on that knife.