"SMH" is an acronym that means "shaking my head" or "shake my head." It can also mean "scratching my head" or "scratch my head."
"SMH" is used when texting and when communicating on Facebook or in chats and forums online when the user finds something so unbelievably ridiculous, disappointing or outlandish that he can only shake or scratch his head. Those who like to sprinkle their communications with profanity use the variations "SMFH" or "SMMFH." Other variations make it more clear why the user is shaking his head; SMHID means "shaking my head in disbelief," and "SMHL" means "shaking my head laughing."