Ainu is a slur used to describe Japanese aboriginals in a derogatory way. Originally, a word for the native Japanese islanders from Hokkaido, but now means roughly a "primitive" person. Usually look more European than their Japanese counterparts, i.e., more hairy and fair skinned. Not really derogatory.
The word Ainu is a racial slur that has been used historically to degrade and dehumanize japanese aboriginals. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to japanese aboriginals. If you are unsure about whether or not it is appropriate to use the word Ainu, it is best to err on the side of caution and avoid using it in any situation.