Isaac is a slur used to describe Jews in a derogatory way. Used by Errol Flynn in a 1933 letter to German intelligence agent Hermann Erben. Assumes that "Isaac" and all of its other incarnations are common names among Jews.
The word Isaac is a racial slur that has been used historically to degrade and dehumanize jews. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to jews. If you are unsure about whether or not it is appropriate to use the word Isaac, it is best to err on the side of caution and avoid using it in any situation.