Hun is a slur used to describe Germans in a derogatory way. When the Europeans united to crush the Chinese boxer rebellion, Kaiser Wilhelm II said something like "let the Germans strike fear into the hearts, so hell be feared like the hun!" Europe laughed, name stuck.
The word Hun is a racial slur that has been used historically to degrade and dehumanize germans. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to germans. If you are unsure about whether or not it is appropriate to use the word Hun, it is best to err on the side of caution and avoid using it in any situation.