What does the abbreviation "SMH" mean in texting?

The acronym SMH means 'shake my head' or 'shaking my head.' It is often used when someone finds something incredibly stupid, ignorant, or disappointing. Although somewhat less often, SMH can also stand for 'scratching my head,' referring to confusion over something someone said or did.

Tag: smh 
Wednesday, February 01 2012
Source: http://www.urbandictionary.com/define.php?term=smh