I did a speech on slang for my high school english class sophomore year (over 20 years ago) and "finna" was one of the words I talked about.
I remember mentioning that it came from fixing to----->"fixin' ta"----->finna
I almost mentioned "hella" which would get you laughed at in southern cali (because it started here in norcal)....now everyone says it...ive even heard a few east coast rappers say it.
stuff i hear from youngsters now a days sucks though:
lit
dead-ass
fleek
bet
i still use hella, dope, yo and gangsta. lol