Getting ready to (do something). Pronounced "fixin ta" or when contracted further "finna". There are phonemes not used in standard American English in these pronunciations so these translations are not completely accurate. Often thought to be limited to Southern white speakers (fixin' to) or African American speakers (finna) the phrase has made it into Standard American speech as "fixing to".
I fixin' to hit you. I'm fixin' to go shopping.
I finna hit you. I finna go shopping.