Born and raised NYC boy here for my first 57 yrs.
Never belonged to a “gang”
“Daddy-o” was an expression used in mid-late 50s
“Flabba” whatever was not. However, neither was “Krup you!” Audiences at that time knew and understood that the language used on the street could not be put on the screen. So other made-up words were used in their p!ace. Lenny Bruce was arrested for “obscenity” in late 1961! You have to understand the times which obviously you do not.
Once again I say, IMO, you are merely using the words of published reviewers and have me questioning myself for ever bothering with you in the first place.
Arthur Laurents made up a slang patois for the book which was always awkward. The new film finds a naturalistic medium that works much better.