SMDH is an acronym for, "Shakin' my dickhead." Used when you witness an event that leaves you speechless. Or used as a follow up dance subsequently after the helicopter.
Example1: I saw this guy driving down the wrong side of the road today. Smdh shakin my dickhead
Example2: I got out of the shower at my girls house. She was watching american idol in the bedroom. So I stood in front of the tv, dropped my towel, smdh shakin my dickhead and broke out the helicopter on her.