Red Coat is a slur used to describe British in a derogatory way. Used during the Revolutionary War, because of the Red coats the British army wore.
The word Red Coat is a racial slur that has been used historically to degrade and dehumanize british. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to british. If you are unsure about whether or not it is appropriate to use the word Red Coat, it is best to err on the side of caution and avoid using it in any situation.