This is a troll swearword and denigration used about another troll who is dislikeable or is, for example, arresting them. In accordance with all things trollish it relates to rock. Coprolite is a Roundworld word, and means "fossilized animal dung". Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name is derived from the Greek words κοπρος / kopros meaning 'dung' and λιθος / lithos meaning 'stone'.
Sometimes used in conjunction with the epithet "Sedimentary", as in "rocks that are formed from sediment left by water, ice, or wind," i.e. created from rubbish. This is one of Detritus' favourite words when describing sellers of Slab, Scrape, Slice and anything else beginning with 's' that makes a troll brain sparkle.
Both words are thus hugely pejorative, so to be called a "sedimentary coprolith", as Brick was, means that he was held in very low esteem indeed.