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."