chewing the invisbile gum
When someone is so wired from crank/crack/meth/diet pills or any other form of speed that they move their mouths as if they were chewing gum, but there is nothing in their mouth. This action is usually accompanied with darting eyes and extreme paranoia.
Johnny was so tweaked that he just sat in his bedroom looking out the blinds and chewing the invisbile gum He has been up for three days and thinks the cops are after him.