Definition of SMH in US English:

SMH

abbreviation

informal
  • Shaking (or shake) my head (used to express disapproval, exasperation, frustration, etc.)

    ‘They'll do anything for ratings. SMH’
    ‘SMH at your silly remarks’
    ‘I can both understand the outrage and SMH at it’
    • ‘Smh. I'm going to need for her to know that she can't go around saying things like this.’
    • ‘That man is father of her children, and all she wants to do is embarrass him in front of them. SMH.’
    • ‘It likes she does it for attention, smh.’
    • ‘SMH at this nasty show.’
    • ‘You let the fake rumours influence your perception of him. SMH.’
    • ‘The sequels sucked so what makes them think a tv series will work! Smh.’
    • ‘Wow smh, so many off-subject arguments and comments.’
    • ‘SMH at the people who post videos on this site.’
    • ‘Right now I'm so ashamed of my home state, smh.’
    • ‘Smh @ her latest selfie.’

Origin

Early 21st century: sometimes also interpreted as short for so much hate.