• To hack something is to cut or slice at it without care to precision. For example, you could wield a weapon and hack a monster. See melee.
• The game of Hack was a clone of Rogue named for the gameplay of hacking monsters. This game is the direct ancestor of NetHack. Jay Fenlason started this game; see Jay Fenlason's Hack. Andries Brouwer made several changes and distributed the game over Usenet; see Andries Brouwer's Hack. It was Mike Stephenson who added the "Net" prefix to distinguish the later game.
• The Jargon File approximates a hack as "an appropriate application of ingenuity". This term is popular among computer programmers who hack on source code to make it do something. The end result of hacking on NetHack is often a patch.

References: hack (dict.org), The Meaning of 'Hack' (Jargon File 4.4.7).

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