gründlich — thorough. The word comes from Grund, the ground. To be gründlich is to go down to the bottom of a thing.
saubere Arbeit — clean work, the common praise for a job done well.
handwerklich sauber — clean in its craft. It is said of a law, an article or a piece of engineering when the work was done properly. A law can be handwerklich sauber and still be politically wrong.
schlampig gearbeitet — sloppily done. The verdict is about the method. It can apply even when the result turned out fine.
Pi mal Daumen — pi times thumb, a rough estimate. The speaker uses it to mark a number that was not properly worked out.