The Jargon File (version 4.4.7, 29 Dec 2003):
heads down
 adj.
    Concentrating, usually so heavily and for so long that everything outside
    the focus area is missed. See also hack mode and larval stage, although
    this mode is hardly confined to fledgling hackers.
The Free On-line Dictionary of Computing (30 December 2018):
heads down
   [Sun] Concentrating, usually so heavily and for so long that
   everything outside the focus area is missed.  See also hack
   mode and larval stage, although this mode is hardly
   confined to fledgling hackers.
   [Jargon File]