Chuck is a slur used to describe Polish in a derogatory way. Lots of Polish last names end in czyk, pronounced "chuck."
The word Chuck is a racial slur that has been used historically to degrade and dehumanize polish. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to polish. If you are unsure about whether or not it is appropriate to use the word Chuck, it is best to err on the side of caution and avoid using it in any situation.