I recently had a friend text me "ima go to [the local pub]. U in?" I understood it just fine as meaning "I'm going to", but being an old fogey, I'm not sure if this is just txtspk or if ima is something people actually say.
Is this just a texting abbreviation? If people really say this, how common is it and where is it mostly used? For instance, can I say it at the office, like "Ima get the spec to you by Thursday." Or is it more casual than that?
It's mainly a spoken form which belongs to some dialects, but not to standard English. It's definitely informal, and I'd suggest not using it in your office example. In fact, I wouldn't recommend using it at all, except to transcribe the word when someone says it.
Is it an abbreviation? Yes. It's usually short for "I'm going to", though in the phrase "I'mma gonna" I think it's a replacement for "I'm". Usage and spelling vary considerably.
EDIT: See comments; apparently some people consider it a standard contraction in spoken English, though I disagree. Regardless, it's definitely non-standard to write it this way.
It is used all the time in informal spoken English. I don't notice whether people say "I'm-a go," "I'm-unna go," "I'm gonna go," or "I'm going to go" when I am talking to them, but they all sound perfectly normal to me. I know I probably use each of those frequently. Of course, you can only use it when it is expressing future tense and not other uses of the words "going to," so, as an example, you can say, "I'm-a think about it," to mean, "I'm going to think about it," but you couldn't say "I'm-a the party" to mean "I'm going to the party."
And although I would never use it when texting, I know some people do all the time. I really dislike "text-speak."
It would sound weird to me to hear "I'm-a gonna," though. I would never use that and I've only heard it when someone was mimicking a Southern or Italian accent.
I first heard Dixie Carter say "Ima" on "Designing Women", and sometimes it would be a bit longer, as "I'monna", which is more obviously shortened from from "I'm gonna", which of course was shorted from "I'm going to." I always say "I'm going to" and do not shorten it. I've said it that way all my life and see no need to change. Also, if you are speaking with someone who has ESL, it makes it easier for them to understand. This construction is considered the progressive tense (using the verb "to be"). It is even more common in Spanish as "Voy a..." in which case it is a form of the future tense, denoting a bit of uncertainty, especially in Latin America, accordining to my Spanish teacher from Guatemala.
You asked how common it is and no one else has given location information, so I will.
I'm in the intermountain west area of the US, and around here it's extremely common to say "I'mma" or "I'monna". In speaking it's considered casual, but not so casual I wouldn't use it when talking to a coworker, and I probably wouldn't even notice if a coworker used it with me unless I was specifically looking for it. I would try to avoid it in more formal contexts, such as a presentation at work. However, in places where it's not common, its use would probably sound "rural" or "hickish".
In writing, it's extremely informal. I would never use it in an email, and in text only with my close friends and even then only when I'm being purposely informal.
