Simple Definition:

“SMH” stands for “shaking my head” and is often used to express disappointment, frustration, or disbelief in response to something someone says or does online. [Disappointment: feeling unhappy because something is not as good as expected; Frustration: feeling annoyed or angry because something is not going as planned; Disbelief: feeling unable to accept that something is true].

Example:
Person A: “I can’t believe she said that.”
Person B: “SMH, some people have no filter.” [Filter: the ability to control what one says or does]

Very Simple Definition:

“SMH” means that someone is shaking their head to express disappointment, frustration, or disbelief.

Example:
Person A: “He didn’t show up to the meeting.”
Person B: “SMH, that’s not professional.”