In response to my post on I'ma, Carrie Shanafelt wrote in to point out that for some people this may be related to another reduced form of "I'm gonna":
In my (caucasian) family this is used as the more immediate version of "I'monna," a Deep-Southernism in which the "g" of "gonna" is dispensed with. As in:
(Rising from the couch, putting on shoes) "I'ma go run."
Response: "See you later!"
(Wearing pajamas) "I'monna go run." (More often: "I'monggo run.")
Response: "Yeah, right."
I don't think I'monna can be exclusively a Deep-Southernism, because it's the normal casual form of "I'm going to" in my own speech, and I was born and raised in rural New England. However, the further reduction (if that's what it is) to "I'ma" isn't a form that I use, as far as I know.
Carrie's note continues
It might be Caribbean in origin, but it's widespread among plenty of white Southern Americans as well as (especially young) African Americans throughout the U.S. "I'monna" (spelled "Ahmowna") used to be one of Jeff Foxworthy's Redneck shibboleths.
Now I'm really curious about this. Given that the I'monna form was normal in east-central Connecticut in the 1950s, at least judging from my own speech and what I remember about that of my kid friends, I'd be surprised if it had any crucial connections to the Caribbean or the American South at all. Of course we pronounced it something like [ˈɐɪ.mə.nə], not [ˈa.mo.nə] or whatever Foxsworthy meant by "ahmowna". And like I said, I come from rural New England, so maybe we should have passed the redneck test after all.
Carrie closed with an interestingly nonstandard idea. It's routine to worry that modern media are homogenizing speech -- or to insist that local forces are resisting successfully -- but she suggests that media are actually serving as new channels to create differentiation, though by subculture rather than by geography:
Posted by Mark Liberman at July 3, 2005 07:29 PMA lot of hiphop slang now, St. Louis-style ("Right thurr!") and Atlanta crunk ("I'ma..."), seems to be popularizing quirks of local dialect that then appear in strange places like New York City. I don't hear older African-Americans in NYC saying "I'ma," just hiphop kids. I guess I'm resistant to the notion of a singular African American Vernacular English in a moment in which there are so many AAVE's being creatively played with, manipulated, exaggerated, and disseminated through music and television.