Swamp Yankee is a slur used to describe Americans in a derogatory way. Of the same nature as "trailer trash", but referring to a poor Whites who live in the boonies.
The word Swamp Yankee is a racial slur that has been used historically to degrade and dehumanize americans. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to americans. If you are unsure about whether or not it is appropriate to use the word Swamp Yankee, it is best to err on the side of caution and avoid using it in any situation.