Finna is a marker in African American English that typically places an action in the near future. Depending on the context, “about to” or “going to” can convey a similar meaning.[1]
Usage
“I’m finna leave” says the departure is imminent. If it’s been thirty minutes, ask whether the speaker has found their other shoe.
Grammar
Linguists discuss finna alongside Southern fixin’ to. The forms overlap in meaning, and their exact historical and grammatical relationship is still a subject of research.