Source : WordNet®

hack on
     v : fix a computer program piecemeal until it works; "I'm not
         very good at hacking but I'll give it my best" [syn: {hack}]

Source : Free On-Line Dictionary of Computing

hack on
     
        To {hack}; implies that the subject is some pre-existing hunk
        of code that one is evolving, as opposed to something one
        might {hack up}.
     
        [{Jargon File}]