Rock stacker is a racial slur against Arabic people. It is insinuating that they are a primitive people that haven't advanced out of the middle ages. I hear it at least once per week. I am surprised it hasn't already been submitted.
I saw a bunch of rock stackers talking rock stacker at the Quik-E-Mart so I turned off my cell phone so I wouldn't detonate any I.E.D's they had.