Rosbif is a slur used to describe British in a derogatory way. Rosbif has been a reference used by the French for the British for hundreds of years - long before tourist holidays. It is a reference to our eating habits, used as a reply to us calling them "frogs" after their habit of eating frogs legs.
The word Rosbif 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 Rosbif, it is best to err on the side of caution and avoid using it in any situation.