<jargon, programming> Sufficiently patched, kluged, and tweaked that the surgical scars are beginning to crowd out normal tissue (compare critical mass). Not all programs that are hacked become "hacked up"; if modifications are done with some eye to coherence and continued maintainability, the software may emerge better for the experience.
Contrast hack up.
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Nearby terms: hackathon « hack attack « hacked off « hacked up » hacker » hacker ethic » hacker humour