Back twenty years ago when I first started seeing "SMH" it was "So. Much. Hate."
As in "I really dislike that."
As in "So, everybody unanimously said they want sheet cake for office birthdays and management decided to double down on the day old donuts again anyway? So Much Hate."
I think it was the original meaning when it was brand new and still very niche.
Part of me wonders if it made it onto one of those "Parents! Know these abbreviations!" memes where they purposely list the acronyms incorrectly (LOL--Lots of Love) and then got more popular that way.