The first meaning of SMH is “Scratching My Head”. This is another way of saying, “I don’t understand why you would do that,” or, “That doesn’t make sense.” SMH is most often used as a single, standalone entry when the writer intends to convey confusion.
Example: Your friend sends you a text message to tell you she just bought a Britney Spears costume for Halloween. You reply only with “SMH” (because you can’t understand why your friend would pick this costume.
Here’s the other meaning of SMH: