stands for 'shake my mutha fuckin god damn head' or maybe ' shakING my mutha fuckin god damn head'. used when people think what is going on is so ignorant there is nothing to do but just shake their head. used when they really think this is the dumbest thing known to mankind!

When you eat dirt for the first time and leave your family to build a shack in the woods

Smmfgdh: Branching off of the simple "smh", which is 'shaking my head' for people who aren't familiar, "smmfgdh" is a more dramatic approach and means 'shaking my motherfucking god damn head.' It's more for those very serious moments...like when you're about to bag it, and she ends up smelling like fish. Or when your girl had to run to her car and get her GPS in an attempt to find your schlong. Or when McDonald's gives you mustard when you CLEARLY asked for ketchup. SMMFGDH.