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.