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]