"While stitching a cut on the hand of a 75 year old farmer, whose hand was caught in the drafting gate while working cattle, the doctor struck up a conversation with the old man. Eventually the topic got around to politicians and their role as our leaders. The old rancher said, "Well, as I see it, most politicians are Post Turtles”. Not being familiar with the term, the doctor asked him what a 'post turtle' was. The old rancher said, "When you're driving down a country road and you come across a fence post with a turtle balanced on top, that's a post turtle." The old farmer saw the puzzled look on the doctor's face so he continued to explain. "You know he didn't get up there by himself, he doesn't belong up there, he doesn't know what to do while he's up there, he's elevated beyond his ability to function, and you just wonder what kind of dumb ar*e put him up there to begin with."
(This explanation is from the Facebook page of Michael Trant , a longer version of that found in the Urban Dictionary.)