SMH is an acronym meaning "shaking my head." It is an expression of un-verbalized contempt, used by people highly contemptuous of the world but too stupid to be able to express their disgust in actual words. It is, essentially, the grunt of the ignorant cynic.
Person 1, not a libertarian: Libertarianism is the political philosophy of toddlers. It is the ultimate outcome of a mindset centered completely on one's self, and ignorant of the existence of other humans.
Persion 2, libertarian: Dude, you just don't get it. SMH.