The Free On-line Dictionary of Computing (19 January 2023):
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]
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.