Rooinek is a slur used to describe British in a derogatory way. This derogatory name was used by the Boers to describe the British primarily after the Anglo-Boer war which lasted from 1899-1902.
The word Rooinek 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 Rooinek, it is best to err on the side of caution and avoid using it in any situation.