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

hack mode
 n.

    1. What one is in when hacking, of course.

    2. More specifically, a Zen-like state of total focus on The Problem that
    may be achieved when one is hacking (this is why every good hacker is part
    mystic). Ability to enter such concentration at will correlates strongly
    with wizardliness; it is one of the most important skills learned during 
    larval stage. Sometimes amplified as deep hack mode.

    Being yanked out of hack mode (see priority interrupt) may be experienced
    as a physical shock, and the sensation of being in hack mode is more than a
    little habituating. The intensity of this experience is probably by itself
    sufficient explanation for the existence of hackers, and explains why many
    resist being promoted out of positions where they can code. See also 
    cyberspace (sense 3).

    Some aspects of hacker etiquette will appear quite odd to an observer
    unaware of the high value placed on hack mode. For example, if someone
    appears at your door, it is perfectly okay to hold up a hand (without
    turning one's eyes away from the screen) to avoid being interrupted. One
    may read, type, and interact with the computer for quite some time before
    further acknowledging the other's presence (of course, he or she is
    reciprocally free to leave without a word). The understanding is that you
    might be in hack mode with a lot of delicate state (sense 2) in your
    head, and you dare not swap that context out until you have reached a
    good point to pause. See also juggling eggs.


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

hack mode
deep hack mode

    Engaged in hacking.  A Zen-like state of total
   focus on The Problem that may be achieved when one is hacking
   (this is why every good hacker is part mystic).  Ability to
   enter such concentration at will correlates strongly with
   wizardliness; it is one of the most important skills learned
   during larval stage.  Sometimes amplified as "deep hack
   mode".

   Being yanked out of hack mode (see priority interrupt) may
   be experienced as a physical shock, and the sensation of being
   in hack mode is more than a little habituating.  The intensity
   of this experience is probably by itself sufficient
   explanation for the existence of hackers, and explains why
   many resist being promoted out of positions where they can
   code.  See also cyberspace.

   Some aspects of hackish etiquette will appear quite odd to an
   observer unaware of the high value placed on hack mode.  For
   example, if someone appears at your door, it is perfectly okay
   to hold up a hand (without turning one's eyes away from the
   screen) to avoid being interrupted.  One may read, type, and
   interact with the computer for quite some time before further
   acknowledging the other's presence (of course, he or she is
   reciprocally free to leave without a word).  The understanding
   is that you might be in hack mode with a lot of delicate
   state in your head, and you dare not swap that context out
   until you have reached a good point to pause.  See also
   juggling eggs.

   [Jargon File]

   (1996-07-31)