Canuck is a slur used to describe Canadians in a derogatory way. From Johnny Canuck, emerging in 1869 as a "younger, simpler cousin to America's Uncle Sam or Britain's John Bull." Reborn during World War II as Canada's 'defender from the Nazi menace.' Could only be a slur if you say it the right way.
The word Canuck is a racial slur that has been used historically to degrade and dehumanize canadians. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to canadians. If you are unsure about whether or not it is appropriate to use the word Canuck, it is best to err on the side of caution and avoid using it in any situation.