Sassenach is a slur used to describe English in a derogatory way. Gaelic word for "southerner" or "saxon." Also the Welsh "Sassain." Link
The word Sassenach is a racial slur that has been used historically to degrade and dehumanize english. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to english. If you are unsure about whether or not it is appropriate to use the word Sassenach, it is best to err on the side of caution and avoid using it in any situation.