Despite all the other explanations here I am very sure this popped up when phone autocorrection became a thing on smartphones. For some reason in the beginning the autocorrection on many phones made "finna" out of "gonna".
So this probably came from "I'
m gonna
kick his behind!" which false was autocorrected to "I'
m finna
kick his behind!"
Keep in mind that this is informal and not used by many people on the planet in real life. Except Snoop Dogg, but come on: It'
s Snoop.