Could someone shed light on the derivation of the word "guinea" when used as a somewhat pejorative
or disparaging description of one of Italian descent similar to the use of the word "wop" for instance?

I don't know about the usage you describe. Is it used in any particular place?

You probably know that a guinea was a unit of UK currency - one pound and a shilling -it's still sometimes used for prizes for horse races (100 guineas, for example). I don't know if there would be any link there.

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.

My Shorter Oxford indicates that the word was used (early 19th century) to describe a black person, especially one recently arrived from Guinea or West Africa. By the late 19th century, it was used derogatively (in the US) to describe an immigrant of Italian or Spanish origin. It doesn't suggest any link between the two terms.

Someone must have really loathed the Itals; guinea originally applied to West African slaves and dago initially referred to Spanish sailors.

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