'First of all - and I don't know how to tell you this without hurting you deeply, but first of all - you happen to be white. I'm talkin' "raised on Rapolla Street white", where your mama used to drag you down to St. Casimir's just like all the other little pisspants on the block. Second, I'm also white. Not "hang-on-the-corner, don't-give-a-fuck white," but "Locust Point I.B.S. Local 47 white." I don't work without no fuckin' contract, and I don't stand around listenin' to horseshit excuses like my cousin Ziggy, who, by the way, is still owed money by you and all your down, street-wise wiggas.'
That's the root of all prejudice, right there: the desperate need for there to be someone, anyone, lower than you in the pecking order.
While Frog has observed the people with power and influence in his world and tried to emulate them to the point of hilarity, Nicky is genuinely insulted by the notion of a white man trying to identify himself with a lower status group. To him, it brings the whole tribe into disrepute.
Look at the things he takes pride in: union, union, union. "I don't work without no fuckin' contract." This is the cult of the 'respectable working class', 'I ain't got much, but what I got, I came by honest'; we're good people, the deserving poor, not like those people, the feckless, the workshy, the chavs, the travellers, the not-white. This speech alone gives a good answer to the question of why he and his would-be colleagues, struggling to get more than five days' work a month, don't just look for a job in another industry. When that work, that union, has been such an ingrained part of your family's identity for generations, the inability to move goes a lot deeper than a lack of transferable skills.
He's distinguishing between Good White - union, blue collar, hard-working hard-drinking three generations on the same docks; a bit of stealing or smuggling now and then, but only for the good of the union, to get your girl a nice place - and Bad White: career criminals favouring a particular black street culture over white tradition.
We might be fucked over by the bosses, the politicians, the entire capitalist system, but here you are: as long as there is someone on a lower rung, we've still got our pride.