Maze Dull is one of my favorites. You can call a guy that, and he won't know what it means, which is part of the reason one would use it, in the first place.
In psychologist B F Skinner's experiments, he put rats in a labyrinth, with a piece of food at the other end. The rats that found the food quickly were labelled Maze Bright, and the ones who took the longest were labelled Maze Dull. The maze itself was called a Skinner Box.
Skinner then bred the fastest male and female rats from the Maze Bright group, and the slowest rats from the Maze Dull group, and ran the test again, and once again selectively bred the brightest of the bright, and the dullest of the dull.
After several generations of selective breeding, there was no longer any 'overlapping' group, in that the slowest of the rats with Maze Bright ancestry found the food faster than the fastest of the rats who had Maze Dull heredity.
When someone is referred to as Maze Dull, the inference is that they are stupid in spite of their best efforts, because their potential for intellectual growth has been curtailed genetically. Not only are you calling them stupid, you are saying that they are stupid because all of their ancestors were, as well.
You can probably say all that with a single word in German, that translates to English as hereditary feeble mindedness, like in the 'hare, hunter, field' scene with Maximilian Schell and Montgomery Clift in Judgment at Nuremberg.
