From 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]
From 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.
From 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]