Harley has what’s called a Brooklyn accent. Brooklyn accents are a very specific type of New York Accent and are most commonly found in New York City. It’s stereotyped as a “tough guy” accent. Words are often shortened, looser, and said with a more exaggerated open mouth.
Other attributes to this accent may include making a “t” sound instead of “to”. “Give it t’me.”
That can be seen here when Harley says “I tried t’play by the rules.” and “Society is t’blame.”
The ends of words are also left open a lot of times. “Billionaire” becomes looser without the hard “e” at the end and the speakers mouth wouldn’t tighten to make the ending. There’s an example of Harley demonstrating this here.
There are specific endings that are typically left off. “Ing” as in “running” becomes “in”, “runnin’”. Seen here when Harley says “puddin’” instead of “pudding”.
“Er” typically becomes an “uh” sound. Demonstrated here when Harley says “slamma” instead of “slammer.”
There’s other things but these are the basics.
Sorry I think I answered too in depth.