Meaning

noun a former British gold coin worth 21 shillings

noun (ethnic slur) offensive term for a person of Italian descent

noun a republic in western Africa on the Atlantic; formerly a French colony; achieved independence from France in 1958

originFrom Guinea, the early modern name for West Africa, the coins originally being made of gold from the region, mostly from the 'Gold Coast' (modern Ghana) and used for African trade, and the guinea fowl being found there. Its use as an ethnic slur against Italians may be due to their darker complexion compared to people of Anglo-American descent.

Etymology from Wiktionary, CC BY-SA 3.0