Here's what I found using Google...
Guinea
region along the west coast of Africa, presumably from an African word (perhaps Tuareg aginaw "black people"). As a derogatory term for "an Italian" (1896) it is from Guinea Negro (1740s) "black person, person of mixed ancestry;" applied to Italians probably because of their dark complexions relative to northern Europeans, and after 1911 it was occasionally applied to Hispanics and Pacific Islanders as well.