I've heard this expression in reference to software development a few times and never really knew what it meant. It comes from an approach developed by Lockheed (and particularly one of it's engineers - Clarence 'Kelly' Johnson) used to build it's first jet fighter ahead of schedule and on budget. Johnson pulled together a small, specialist team within the larger organisation and 'flew under the radar' cutting red tape and just getting things done.
37Signals.com have written about it with respect to a philosophy of design.
'Quick prototypes. Small teams. No bureaucracy. No lengthy documentation. Limited meetings'.
So now I know, and can aspire.