This is an Acronym that originally meant "Silently Masturbating During Homework". It was converted into "Shaking My Damn Head" when a Facebook member who often used this term was asked, "What does that mean?". The individual responded with a quick and witty answer. "Shaking My Damn Head". The individual who asked what SMDH means responded, "You sure do shake your damn head a lot".
It means "shake my dick head". But you also have to say "SMDH at you," otherwise, you would just be shaking it for no apparent reason except to get off. So, it basically means the same thing as the original except more of a tisk tisk with exposed genitalia.
Dude 1-"Man, I lost that chick's number from the other night!"
Dude 2-"Sigh...smDh at you."