From Discworld & Terry Pratchett Wiki
As a troll, retrophrenology isn't likely to hurt Zorgo. The idea, clearly invented in literal-minded Ankh-Morpork, is that if lumps and bumps on the head can be used to tell the characteristics of an individual then, logically, if you cause lumps and bumps on the head (by subtly whacking it with a graded series of mallets) you will develop that aspect of someone's character.
For more, see the main article on retrophrenology.
Whilst at first glance this seems to be the first non-mineral-based troll name on the Disc, actually zorgite is a metallic copper-lead selenide, found at Zorge, in the German Harz Mountains.
In the first Dirty Harry movie, Harry has a shoot-out with an unseen assailant that leads to a rooftop chase, in which he crashes through various doors, skylights and windows in single-minded pursuit of his quarry, blithely ignoring the privacy of the occupants, but calling over his shoulder "Just carry on with what you're doing, OK?" as he forces his way out again. The private doings that Dirty Harry intrudes on while in hot pursuit include the making of a porn movie, and (as I remember) a dentist working on a patient's mouth.