Illinois prison slang for a new inmate. If you make a rookie mistake, someone might try to cover for you by saying. "He's a'ight. He's on the new."
Alternately, it's used as a term of derision when applied to someone who is not "on the new". For example anyone who drops the tray in the chow hall is going to be greeted with a chorus of "On the new! On the new!".