WordNet (r) 3.0 (2006):

hack on
    v 1: fix a computer program piecemeal until it works; "I'm not
         very good at hacking but I'll give it my best" [syn:
         hack, hack on]

The Jargon File (version 4.4.7, 29 Dec 2003):

hack on
 vt.

    [very common] 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.


The Free On-line Dictionary of Computing (30 December 2018):

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]