(slang) A slow and painful murder by the Mafia as a warning to others.
2000, Olindo Romeo Chiocca, Mobsters and Thugs: Quotes from the Underworld (page 17)
Mafia mythology has many believe that the method used to eliminate a person is often a reflection of the respect for that individual. In this case, the Profaci gunman must have witnessed a "buckwheat's" execution, where an example is being made of the victim by inflicting pain for a prolonged period.
2006, Carl Sifakis, The Mafia Encyclopedia (page 66)
Mob murders are seldom buckwheats, being instead simple business matters. An exception is made, however, for murders of example, such as in the case of informers, or mobsters who hold out on gang revenues […]