I don't know what the origin of the word "guinea" is, but I have heard it used as a slur towards Italians.
It is also used in another way, one that is perhaps related. A "guinea-tee" is a white tank-top undershirt, worn by men. I'm guessing that it is related to the pejorative use of "guinea". I've also heard this undershirt called a "wife-beater." Whether or not these are all related, I don't know.