Keep on 9

To Keep on 9 means to be aware at all times without getting distracted.
By telling someone to keep on 9 is usually a threat made by posing gang members to inform you that you are in danger.

person 1 : Aye suuwhoop watchu doin in my area
person 2 : What’s it to you slob

person 1 : Your ass better keep on 9 cause im about to pump you full of lead