SMH can also stand for "shaking my head." In this sense SMH can be used in the place of words such as disappointed or confused. SMH is the action that sometimes accompanies the feeling of disappointment or confusion.
George: Ok, I'll check for it when I get home.
Persion 2, libertarian: Dude, you just don't get it. SMH.
However, due to the widespread use of "smh" from those who have learned its meaning incorrectly, it can be assumed that many who use it do so in the erroneous meaning of "shaking my head." It can usually be read either way.
"smh" is similar in meaning to "fml", but more angry than despairing.
"The government has instituted a new tax on, get this, twisty-ties. smh"