Just review passenger ship lists. Every once in a while you’ll come across an Italian or Irish immigrant with the handwritten word “WOP” where it is suppose to be confirmed that they are carrying the proper documents to come to the United States.
Then why aren’t the Irish ever referred to with that slur? Guappo seems much more likely.
I was born and raised in Philadelphia. I attended Catholic schools where half the kids celebrated St. Paddy's Day (the Irish) and the other half St. Joseph's (the Italians). It was always a big deal because those two days fell during Lent and yet, depending upon your last name, you had one day when you could eat, drink, and be merry. In all my years, I have never once heard someone of Irish extraction referred to as a wop. It was always my understanding that wop was short for guappo, which I think was used in Naples to mean thug.
But what do I know? I'm just a drunken Mick. ;)