I'mma

Most students are familiar with the shortening of 'I am going to' to 'I'm gonna', but did you know that many speakers shorten it even further to 'I'mma'?

So 'I am going to tell him' becomes 'I'mma tell'im'.

As far as I know, this is not in any textbook, but it is becoming widely used. Just another way spoken English is different from written English.