To 'buckweat' someone means to murder them in any method that causes a slow, very painful death. The phrase 'buckwheat' was given rise to in the film ''Things to do in Denver when you're Dead''. (An excellent movie which I highly recommend).
One example of a 'buckweat' is to fire a gun into someone's rectum. This causes them to contort and suffer excruciatingly for 20 minutes before they die.