1) (noun) An object that, while not necessarily originally intended as a weapon (and otherwise bears no resemblance), is used as one. A steak knife, for instance, would not be a shank (and is, as well, a poor ad hoc choice for a weapon), but a screwdriver, heavy duty flashlight, or socket wrench would be.
Origin unknown, but I think it has something to do with home made knives. See also shiv
2) (verb) To use a shank. (Shanked, past tense)
Jorge was shanked with a screwdriver by Miguel when he failed to pass on his protection money.