P-shakes is a term invented and used by Mutt. It is short for "physical shakes" (as opposed to mental ones, 'm-shakes') and refers to physical, macro muscle tremors of the sort that afflict Mutt. They cause no discomfort but can impair manual dexterity severely and are generally impossible to override.
Sometimes p-shakes can be brought on by high negative feelings, and it would follow that at least some of the time the shakes are symptoms of stress. Most of the time, though, they just show up during a high proportion of everyday attempts to manipulate the physical world.