Short for "not going to" when followed by be + -
ing form of a verb, otherwise
short for "wouldn't have/had" or "couldn't have".
a. I'm notta' be leaving for
today.
= I'm not going to be leaving for
today.
b. I notta'
come, but he forced me to, haha!
= I wouldn't have come, but he forced me to, haha!
c. I notta' done it without you.
= I couldn't have done it without you.