Foster is a slur used to describe Australians in a derogatory way. Australian for piss-poor beer that no self-respecting Aussie would be seen dead drinking.
The word Foster is a racial slur that has been used historically to degrade and dehumanize australians. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to australians. If you are unsure about whether or not it is appropriate to use the word Foster, it is best to err on the side of caution and avoid using it in any situation.