Nedarim 11 - You are not kosher to me!
One can use the language of "not kosher" to create a vow. For example, if he says, "It is not kosher what I eat of yours!" - he has in fact forbidden himself the food coming from his fellow, and it would be counted as if he violated the Torah if he did eat it.
He can also say, "It is not regular (chulin) food," or "not clean," or "like the remainder of a sacrifice" (which can not be eaten).
However, watch for provisos and caveats: if he says, "It is not kosher what I do not eat of yours," then he means that what he does NOT eat will be not kosher, but what he DOES eat will be kosher, and thus he created no vow and no prohibition.
Art: Girl Eating an Apple by Godfried Schalcken