DAVINA: “Physic” comes from the Latin “physicum”, and means either a medical remedy, the curing process, or medical science in general. We retain this last scientific sense today when we refer to the study the inanimate world as the subject of physics. 
RALPH: Macbeth is saying here that the delight he gets from hosting the king eliminates any pain that might cause. It seems that in Shakespeare’s time, most “physics” involved some kind of elimination – blood-letting with leeches, or cathartics and purgatives that induced vomiting and bowel movements. 
DAVINA: I think we get the picture, Ralph.