Why does Lefty frequently use the phrase "fuhgeddaboudit"?

In the article linked to by DCdotWEB in the comments, “fuhgeddaboudit” is described as follows:

The elision of "forget about it" is a regionalism that gained popularity in New York and New Jersey. Fuhgeddaboudit is "used indicate that a suggested scenario is unlikely or undesirable," the word's new dictionary entry reads.

It’s one of those terms that can mean whatever the speaker wants based on context. Other examples could include “prego” in Italian, which has at least three or four formal definitions and I’ve seen it used informally in lot more ways, or “cheers” in BrEng, which can mean “your good health”, “thanks”, or “good-bye” amongst other meanings.

It’s nicely summed up by Donnie himself in this video.