In order to understand this question, two words need to be defined: euphemism and eugenics. A euphemism is a more acceptable (or gentle) term for something harsh or offensive. Eugenics is the belief that one can improve humanity by encouraging people with desirable traits to reproduce while discouraging people with undesirable traits to reproduce. The former is called positive eugenics while the latter is called negative eugenics. Hitler was a fan of both in that he used positive eugenics to encourage the blond-haired and blue-eyed Germans to reproduce and used negative eugenics to discourage the dark-haired and dark-eyed Jews from reproducing.
Because eugenics is an offensive concept, euphemisms are used throughout The Book Thief. One of the ways Hitler encouraged eugenics was to force young people into his group called “Hitler’s Youth.” In the novel, the author states that for girls like Liesel, “Hitler’s Youth meant a small brown uniform.” This is a euphemism. What it really means is the brainwashing of girls like Liesel into believing that their race was better than the Jewish race. Another euphemism for eugenics has to do with the minor character of Tommy Muller. Liesel states the following:
[Tommy’s] ear infections and nerve damage were still contorting the marching pattern of the Hitler Youth, which, I can assure you, was not a positive thing.
These words by our narrator, especially the words “not a positive thing” are euphemisms. We can tell this because of the reaction of the Hitler Youth leader. Due to Tommy’s twitching and his inability to start and stop marching in time, the leader always yells, “You ape—what’s wrong with you?” This term (which is not a euphemism) shows the Nazi intolerance for anyone with a disability (which rivals the Nazi intolerance for anyone of a different race). Euphemisms for intolerance continue throughout the entire novel.
In conclusion, the reader can see that the euphemisms given here are designed to show that the Hitler Youth organization and its ultimate leader, Adolf Hitler, are the true enemies. The brown uniform and the disabled boy are not the real problems.