A Guinea is still used in British Horse Racing, and is of the value of one pound and five pence.
The word Guinea is associated with the coin/currency unit in the UK rather than as a reference to Italians.
Unlike the "N-word", which African-Americans sometimes use to address each other, no Italian-American would ever address another Italian-American using this word. Nor would they use the word "wop" (also offensive, but not in a racial way). HOWEVER, it IS common for Italian-Americans to refer to each other as "dago"; this is used the same way that blacks use the N-word with each other.
Guineas and Guidos are NOT the same thing.
People usually say, Guidos are Italian. Not true. Guidos are basically wanna-be Italians. Men that dress like the stereotypical Italian or Italian American just to receive recognition by others. Many people tend to call Italians "Guido". Best bet is to just not call an Italian a Guinea if your NOT Italian, unless you want a pissed of Italian wanting to kick your ass.
Salvatore : Hey there you damn Guinea. How you holdin' up?
Roberto : Pretty good... You're self?
A Non-Italian Speaking to an Italian.
Bob: Hey ya guinea! Hows it going?
Roberto: Call me a guinea again and I'll push those words out your ass.
Refrencing the belief that Southern Italians recieved darker genes from the Moore colinization of Sicily, and the similar appearence to people from the Guinea coast of Africa.
It is often interpreted as being called part-nigger and non-white and is usually taken very offensively.
Guido: Yeah right, paddy!