A homemade weapon usually made out of old crap lying around like broken glass or metal. A shank usually is only for stabbing and has no cutting edge (like a dart, but a lot bigger and more dangerous).
As a verb shank means to stab somebody to death with a shank, with quick short motions to the area just below the ribs.
-In both of these senses shiv means the same thing.
"Shank" can also be used in golf to describe a terrible slice or duff resultant from the club head not centered on the ball during the swing. Also a verb.
I've also heard from a guy in NC that they use "shank" to mean pantsing where he lives. Hardly anybody else I've asked says this is true, though. Must be a southern thing as far as I know. Usually they yell "shank" as they pull down the pants.
-I crafted a shank and shanked this guy in prison.
-Ahh! I shanked that drive!
-"SHANK!" he yelled as he pantsed Robert.