"cough syrup" refers to cough syrup mixed with hydrocodeine. This combination, when mixed with sprite is called lean, purple drank, and syrup. It's heavy narcotic effect relaxes your muscles, hence you move/talk slow.
in old westerns (movies set in the 1800s in the american west) cowboys shot at someone's feet to get them to "Dance, boy!" - a way of threatening them and showing disrespect. this song is giving you the urge to 2-step as if your life depended on it.