Kees is a slur used to describe Dutch in a derogatory way. Flemish for "cheese". Also a very common name in Holland (and possibly also Flanders); it is a shortened form of the name "Cornelis". It is sometimes spelled "Cees".
The word Kees is a racial slur that has been used historically to degrade and dehumanize dutch. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to dutch. If you are unsure about whether or not it is appropriate to use the word Kees, it is best to err on the side of caution and avoid using it in any situation.