What will be the first 3d printed "weapon" that will actually be used?

There is lot of hype about 3d printed guns and predictions that we will see first major crime committed with them in 2014.
Maybe. But if had to take a guess, I would guess that the first 3d printed "weapon" to be used will be some sort of brass knuckles or knuckle duster.

Yeggi lists 416 3d models of various kinds of "brass knuckles", so they are widely available on all public repositories.

http://www.yeggi.com/q/brass+knuckles/?s=tt

Some kid somewhere with 3d printer will want to be "cool" or gain some "edge" on some bullies end he (or she) will print it and bring it to school. After that there will be explosion of stupidity and fear mongering.

We will see. There will certainly be a LOT of news coverage even if it results in nothing more than few bruises.