Dyke is a slur used to describe Dutch in a derogatory way. The country owes its existence to the many dikes that were used to reclaim land from the sea, also from the story of the Dutch boy who used his finger to plug a leaking dike to save his town. Van Dyk/Dijk/Dyke is also a very distinctive Dutch last name.
The word Dyke 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 Dyke, it is best to err on the side of caution and avoid using it in any situation.