Buster is a slur used to describe Bulgarians in a derogatory way. From the cartoon Arthur
The word Buster is a racial slur that has been used historically to degrade and dehumanize bulgarians. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to bulgarians. If you are unsure about whether or not it is appropriate to use the word Buster, it is best to err on the side of caution and avoid using it in any situation.