Harbor-Bomber is a slur used to describe Japanese in a derogatory way. The Japanese bombed Pearl Harbor in 1941, which caused the US to enter World War II.
The word Harbor-Bomber is a racial slur that has been used historically to degrade and dehumanize japanese. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to japanese. If you are unsure about whether or not it is appropriate to use the word Harbor-Bomber, it is best to err on the side of caution and avoid using it in any situation.