Frostback is a slur used to describe Canadians in a derogatory way. A play on "wetback", altered to fit Canadians.
The word Frostback 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 Frostback, it is best to err on the side of caution and avoid using it in any situation.