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.