Wearing clothes but no underwear. Also known as freeballing, in
the male case.  No clues as to the origin of the phrase. Maybe
because it requires someone with balls of brass (figuratively
speaking, of course) to do it (blue jeans chafe genitalia
something awful, I would think)? Maybe because commandoes carry
a minumum of gear with them?  My educated lie* about the origin
of the term comes from the Navy SEALs who fought in Vietnam (and
possibly their jungle-warfare predecessors). When you're running
around a hot, sweaty jungle environment for days on end, the
last thing you want is to have any more clothing on than is
necessary--especially an article that is likely to get funky,
and stay that way, for the duration of your trip.  In fact, now
that I think on it, my Dad mentioned that one of the rituals
that his air wing would subject the newbies to was to check to
see if they were wearing skivvies on arrival. If they were, they
would grab the waistband and forcibly rip them off the soon-to-
be-clutching-his-gro