SMH is an acronym for “shake my head” or “shaking my head”. In addition to being used when texting, SMH is also used on social networking websites and similar venues. Some people occasionally use SMH and similar abbreviations in email communications.
SMH typically is utilized when a person wants to convey that something is particularly stupid or unintelligent. The verbal or spoken equivalent is a statement along the lines of "all I can do is shake my head." In highly casual communications, some individuals may interject an additional letter into SMH as a means of essentially injecting profanity into the conversation.