What does smh mean dirty?

Excuse me, I'm a bit perplexed here. Could you please clarify for me what 'smh' stands for when used in a dirty or inappropriate context? I've seen it used in a few online conversations recently and it seems to carry a negative or sarcastic tone, but I'm not quite sure of the exact meaning. Could you help me understand this better? It would be greatly appreciated if you could provide an explanation in a way that's easy to grasp.