Well that’s a different thing, as it’s not an abbreviation, but an acronym. Which is why that actually gave me pause after years of using “smth” online, because I always read “smh” as “something” instead of “shaking my head.”
It always bothered me because “shaking my head” isn’t even a figure of speech people say, so how the fuck can you use an acronym form of the expression.
Much like how gifs work, I presume.
Like you just imagine someone shaking their head. Just kine you imagine someone smiling when I type :)
The thing is, shaking ones head isn’t as universal of a symbol as smiling. Some cultures have nods and shakes the other way around. Which seems somewhat weird.