Derived from the online acronym smh (shaking my head), this phrase describes the action taken by someone who's so disappointed in an event or action, their only sensible reaction is to literally shake their head off of their own shoulders. Not to be confused with lmao (laughing my ass off), which is usually reserved for moments of hilarity.
T_Bird262: i just got back from the best concert of my life!!1
Dave2012: 4real? who u c?
T_Bird262: justin bieberzz!!