Although once thought to describe the act of having a fit (i.e., a seizure), it has evolved into a ghetto way to say "fixin' to, about to, or going to do something."
I ain't fittin to take no spelin class.
Getting ready to do something.
Are you ever going to mow the yard? Ya, I'm FITTIN TO!
Appropriate, suitable, acceptable
Her behavior at the dance ain't fittin.