Fish is a slur used to describe Israelis in a derogatory way. Fucking Israeli Shithead. See: Dib.
The word Fish is a racial slur that has been used historically to degrade and dehumanize israelis. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to israelis. If you are unsure about whether or not it is appropriate to use the word Fish, it is best to err on the side of caution and avoid using it in any situation.