Being executedwith a bullet up the rectum, i.e. a very slow and painful death. Courtesy of Chistopher Walken in Things to Do in Denver When You're Dead.
To tell someone where the bear shit in the buckwheat means to explain something to them that they are very likely to know already, especially because it is self-evident.
«I'm not that stupid that I actually need somebody to tell me where the bear shit in the buckwheat: I can see for myself who the bad guy in the film is. Ta!»
Billy: Did you go downtown last night?
Jim: Yah, but not worth braggin' man that shit was a buckwheat beaver, bro!
Billy: Are you serious?
Jim: Yeah, I'm still pullin pube's out of my teeth!