I’m on [verb]

The English language has no future tense. To refer to the future, we use various strategies with verbs in present tense (some of them auxiliaries):

I will run
I will be running
I shall run
I’m going to run
I am to run next
I’m running tomorrow
I run next Friday

Because we can conceptualise the future and it plays a big role in our lives, we talk about it often. Naturally, then, the ways we talk about it are subject to pressures of economy, resulting in contraction, e.g.:

I will run → I’ll run
I am going to run → I’m gonna/gon’ run
I’m gonna run → I’mna run → I’ma/Imma run

I’ll is acceptable in Standard English; gonna/gon’ and I’mna/I’ma/Imma are not, though you may see them in dialogue or informal writing or use some of them yourself in everyday speech – gonna is especially widespread.

Recently I came across another form: I’m on [verb]. It seems similar to I’ma and I’m gon’, but I don’t know exactly how or when it developed. Here’s the example I saw, in Elmore Leonard’s novel Mr. Paradise: